Displays

How to cite: Lipp, A., Shorttle, O., Syvret, F., Roberts, G., and Sedimentary Geochemistry and Paleoenvironments Project, W. I. W. G.: Deconvolving weathering and provenance in the composition of the modern and ancient continental crust, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5825, https://doi.org/10.5194/egusphere-egu2020-5825, 2020.

Detrital sediments provide a useful tool to investigate the composition of the continental crust through time. Mass-dependent (“stable”) isotope variations in Archaean to present-day sediments (shales, diamictites) have recently received much attention and Ti, in particular, holds significant promise as a novel tracer of crustal composition [1, 2, 3]. This approach is based on i) the contrasting Ti isotope composition of mafic versus felsic rocks as a result of the removal of isotopically light oxides during igneous differentiation; and ii) the chemical behaviour of Ti, a refractory and biologically inert element that should not fractionate during weathering and sedimentation. Hence, current interpretations of the Ti isotope detrital sediment record rely heavily on the assumption that it reflects the integrated composition of the source(s), and thus provides a record of the proportion of felsic to mafic rocks in that source.

A potential caveat, however, is the hydrodynamic sorting of dense minerals in coarse, more proximal sediments [4]. This effect is well-known for zircon; coarser sediments tend to have higher Zr/Al₂O₃ and a less radiogenic Hf isotope composition due to the concentration of zircon grains [e.g., 5, 6]. Shales form the complementary zircon-depleted reservoir characterised by lower Zr/Al₂O₃ and a more radiogenic Hf isotope composition relative to the source. Common Ti-rich phases such as ilmenite and rutile are also resistant against physical and chemical weathering and could be concentrated together with zircon in coarse sediments.

We examined a suite of Eastern Mediterranean passive margin sediments with well-constrained provenance [7] and found that Ti indeed behaves like Zr. Fine-grained samples have lower TiO₂/Al₂O₃ compared to coarser, proximal deposits of identical provenance. The removal of Ti-rich phases with a light Ti isotope composition into coarse-grained sediments could thus bias the Ti isotope composition of shales towards isotopically heavier values. We will report on the δ^49/47Ti isotope composition of these sediment samples, but a TiO₂/Al₂O₃ mass balance suggests that a bias of more than 0.05 ‰ in the δ^49/47Ti of shales is possible. Understanding the consequences of hydrodynamic sorting for Ti isotopes in sediments is crucial for their use as a quantitative proxy of crustal composition and for reconciling the shale and diamictite Ti isotope records.

[1] Greber et al. (2017) Science 357 1271-1274; [2] Deng et al. (2019) PNAS 116-4 1132-1135; [3] Saji et al. (2019) Goldschmidt abstract 2929; [4] Greber & Dauphas (2019) GCA 255 247-264; [5] Patchett et al. (1984) EPSL 69 365-378; [6] Carpentier et al. (2009) EPSL 287 86-99; [7] Klaver et al. (2015) GCA 153 149-168.

How to cite: Klaver, M., Vroon, P., and Millet, M.-A.: The effects of hydrodynamic sorting on the Ti isotope composition of sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9329, https://doi.org/10.5194/egusphere-egu2020-9329, 2020.

When subduction initiated and contributed to formation of Continental crust is uncertain. A crucial difference between subduction zones magma and e.g. plume related magmatism is the role of H2O in the magma formed. Subduction zones magma are frequently wet and follow a liquid line of descent (LLD) that differs from dry plume related magmas. We developed a qualitative hygrometer based on major elements that allow to distinguish between LLD formed at water saturated condition from those that formed at dry conditions. While arc magmas can by dry at times, plume related magmas are generally dry. So wet LLD are a hall mark of subduction. In this talk we will compare the modern arc record with the Archean rock record to investigate if Archean rocks formed due to a wet or dry LLD. 

How to cite: Jagoutz, O., Klein, B., Schmidt, M. W., and Küter, N.: Continental Crust formation in the Archean vs modern times, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5997, https://doi.org/10.5194/egusphere-egu2020-5997, 2020.

Tracing the origin and evolution of magmas on their pathway through the lithosphere is key to understanding the magmatic processes that eventually produce eruptions. For ancient magmatic provinces, isotope-geochemical tracers are powerful tools to probe the source regions and magma-crust interaction during ascent and storage.

We present new hafnium isotopic compositions of ID-TIMS dated zircons to trace the evolution of the Middle Triassic magmatic province in the Southern Alps (northern Italy) at high temporal resolution [1]. Systematic changes in hafnium isotopic composition with time reveal a coherent temporal evolution from depleted mantle signatures towards crust-dominated signatures within less than four million years. This trend can be ascribed to progressive influence of a crustal source, incorporated into the reservoir from which these zircons crystallized. Towards the end of the magmatic episode, the εHf compositions abruptly revert within one-million-years back towards more juvenile compositions mainly recorded by the mafic to intermediate intrusive pulses (e.g. Monzoni and Predazzo), the effusive climax of basaltic lavas and the post-intrusive ash beds (e.g. Punta Grohmann) in the Dolomite region. We interpret the variation of Hf-isotopic signatures over time as a protracted contamination signal induced by interaction of the mantle-derived magmas with the lower crust.

The dataset obtained in this study is further implemented into a two-component mixing model employing a range of potential crust and mantle endmember Hf isotope signatures and Hf concentrations which is directly translated into crustal melt/total melt (=sum of crustal and mantle-derived melt) ratios over time. Based on these observations we explored the thermal evolution and crustal melting as a function of time, lithology, water content and magma flux for a lower crustal magmatic system by numerical modelling. Dykes and sills of basaltic composition are incrementally emplaced at the mantle-crust boundary, which leads to changes in crustal over mantle melt ratios over time. Initial intrusions of basaltic dykes into the relatively cold lower crust cause only limited crustal melting and assimilation but ensuing magma injections into progressively hotter crust results in more extensive partial melting and assimilation of crustal material. Subsequent intrusions into the magmatic lower-crustal roots cannibalize previous intrusions with progressively less isotopic contrast due to dilution with mantle-derived magmas. This is potentially accompanied by an increase in magma flux, e.g. by delamination of dense lower crustal cumulates into the subcontinental lithospheric mantle.

The observed trends in hafnium isotopic composition therefore do not necessarily require tectonic re-organizations or changes in mantle sources. Instead these trends may trace variations in mantle-crust interaction during thermally induced chemical maturation of the lower crustal magmatic roots progressively replacing ancient pelitic to mafic lower crustal lithologies by juvenile cumulates.

[1] Storck, J.-C., Wotzlaw, J.-F., Karakas, O., Brack, P., Gerdes, A., Ulmer, P. Hafnium isotopic record of mantle-crust interaction in an evolving continental magmatic system, Earth and Planetary Science Letters, (in press).

How to cite: Storck, J.-C., Wotzlaw, J.-F., Karakas, O., Brack, P., Gerdes, A., and Ulmer, P.: Hafnium isotopic record of crustal maturation during Middle Triassic magmatism in the Southern Alps (Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14303, https://doi.org/10.5194/egusphere-egu2020-14303, 2020.

Tonalite, Trondhjemite, Granodiorite (TTG) rocks in Viti Levu, Fiji islands formed through hydrous melting of gabbroic oceanic crust at low-pressure amphibolite-facies conditions caused by flat subduction of an oceanic plateau from Yavuna creek. During mid Miocene time, magmatic underplating took place and a Qtz-diorite unit was formed out of the gabbro under granulite-facies conditions. The investigated TTG´s occur as stocks and veins within the older gabbroic unit of the Yavuna Pluton.

Zircon ages show the parental gabbro to be ~47.5 Ma in age, whereas the TTG´s, which can be subdivided into a tonalite and a Qtz-diorite suite, are ~37.1 Ma and ~16.5 Ma, old respectively. The average d¹⁸O value of ~4.8 in zircon selected from the parental gabbro and the tonalite suggest a very homogenous mantle source. However, about 50% of the analyzed zircons from the gabbroic and tonalitic rock samples showing lower d¹⁸O values, and these are interpreted as reflecting interaction of hydrothermally altered seafloor with the deep depleted mantle source. eHf in zircon values of ~13 in the analyzed TTG´s are interpreted as reflecting typical juvenile continental crust. PerpleX whole-rock calculations suggest that the tonalite formed by melting of the gabbro through decompression under water-saturated amphibolite-facies conditions at a temperature of ~770 °C and a pressure of ~3.8 kbar, whereas the Qtz-diorite formed at a temperature up to ~900 °C at very shallow depth close to the Earth’s surface caused by the emplacement of a magmatic underplate during the mid Miocene. Our investigation provides new evidence for episodic growth of continental crust < 0.1 Ga in the South Pacific region.

How to cite: Sommer, H., Kröner, A., Jacob, D. E., Che, X., Wong, J., and Xie, H.: Cold avalanche, “super subduction”, mantle overturn, followed by buoyant subduction of an oceanic plateau and the formation of TTG´s during the Eocene in Viti Levu, Fiji islands, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10410, https://doi.org/10.5194/egusphere-egu2020-10410, 2020.

Neoarchean granitoid gneisses are widely distributed throughout Eastern Hebei, eastern North China Craton, and are dominated by deformed and metamorphosed tonalite–trondhjemite–granodiorite (TTG), diorite, and granite. This study presents the results of systematic zircon U–Pb geochronological, whole-rock geochemical and Sm–Nd isotopic analyses of the Neoarchean granitoid gneisses in Eastern Hebei. These data provide insights into the Archean–Paleoproterozoic multiple tectonothermal events and the petrogenesis of the gneisses in this area. U–Pb ages and cathodoluminescence images of zircons from the granitoid gneisses using laser ablation-inductively coupled plasma-mass spectrometry (LA–ICP–MS) indicate that their magmatic precursors were contemporaneously emplaced between 2546 ± 10 and 2510 ± 10 Ma, reflecting a giant Neoarchean igneous event throughout Eastern Hebei. Subsequently these rocks were subjected to regional amphibolite facies metamorphism at 2.48 – 2.45 Ga. The close spatial and temporal relationships between magmatism and metamorphism at ca. 2.5 Ga suggest a uniform tectonothermal evolution of Eastern Hebei. The granitic gneisses are considered to have mainly originated from the partial melting of juvenile metamorphosed greywackes, with minor involvement of basalts. The large geochemical and isotopic variations within the dioritic and TTG gneisses both provide evidence for the mixing of mafic and felsic magmas, coupled with fractional crystallization. However, the chemical differences between the dioritic and TTG gneisses might be because they originated from different mafic magma sources, viz., basaltic and high-Mg melts. The mafic magma may have also formed the metamorphosed basalt or komatiite within the greenstone belt or evolved via fractional crystallization prior to the magma mixing. Large-scale granitoid activities were possibly related to mafic magma underplating. The combined geochronological, geochemical, and geological data support an Archean proto-mantle plume model for interpreting the geodynamics of the eastern North China Craton during the Neoarchean.

Acknowledgements Our work was supported financially by Beijing Natural Science Foundation (Grant Number: 8194073), the Science Foundation of China University of Petroleum, Beijing (Grant Number: 2462017YJRC032) and the Science Foundation of State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing (Grant Number: PRP/indep-4-1702).

How to cite: Li, Z. and Zhang, W.: U–Pb–Hf–O–Nd isotopes and geochemistry of the Neoarchean granitoid gneisses in Eastern Hebei, North China Craton: Implications for crustal growth, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22081, https://doi.org/10.5194/egusphere-egu2020-22081, 2020.

The information on magmatic orthopyroxene (opx) granitoids (charnockites) is essential for geodynamic models of continent collision, amalgamation, and postorogenic events. There is also the petrological issue - the role of the crust and mantle material in the formation of opx granitoids in orogenic and post-orogenic settings. Our research objects are three multiphase coeval plutons of the opx granitoids in the Banger Oasis, East Antarctica. Only two phases can be distinguished: the first phase comprising of silica and potassium rich middle-coarse grained qz opx monzodiorites, monzonites and granites with porphyric perthite alkali feldspar, whereas the second phase is more basic and comprises of alkali feldspar less fine-middle grained qz gabbroes and qz-opx diorites and gabbro-diorites with gabbroic and ophitic microstructure.  These plutons intrude granulite facies metamorphic bedrock. The age of those plutons is around 1170 ± 10 Ma. The first phase has the age around 1170-1190 Ma, whilst the second phase has the age of 1150 1170 Ma. The interval of peak metamorphism is constrained by 1250-1170 Ma. We combine together thermodynamic modelling and geochemistry (isotope geochemistry in Sm-Nd, Rb-Sr and Pb-Pb systems, Hf and O stable isotopes data on zircons) to create a petrological model for opx granitoids formation. As a result of such modelling we are able to prove that pluthons were crystallized at grnulitic facies and dry conditions. Within the first phase of the crystal fractionation the process is traced. These phases have also different isotope characteristics. For example, the ε(Nd)₀ is around -7 to -12 for the second more basic phase and – 16 to -22 for the first opx granitoids phase. It is assumed that the structures of the Banger oasis are the result of the reworking of the Yilgarn craton in the Early Proterozoic. Afterwards there was a collision and amalgamation of the supercontinent Rodinia, and at the final stage, the intrusion of opx granitoids occurred. We consider a presence of mantle material in the formation of charnockite melts. It is possible that the differentiation of tholeiitic magma and the mixing with the crustal component took place.  That might also go along with delamination of the thickened continental crust after the completion of collisional orogeny. Mixing with mantle material could occur at lowest levels of the crust and afterwards the mixed melt moves consequently to the higher levels and differentiates in chambers. Last phases are more enriched in mantle component according to modelling and geochemistry.

How to cite: Borovkov, N., Kusiak, M. A., Nawrot, A., Koreshkova, M., and Leichenkov, G.: Origin and Geodynamic significance of opx granitoids from Bunger Hills Area, East Antarctic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1051, https://doi.org/10.5194/egusphere-egu2020-1051, 2020.

We present zircon U-Pb ages and whole-rock geochemistry along with mineral chemistry of the Khardung volcanic rocks outcropped in the northern margin of the Ladakh batholith in order to constrain their origin and tectono-magmatic history. These volcanic rocks are sandwiched between the Ladakh batholith in the south and the Shyok suture zone in the north and span a continuous compositional range from basalt to rhyolite, although mafic rocks are minor and intermediate to felsic rocks are volumetrically predominant. New zircon U-Pb dating for andesite coupled with two rhyolitic rocks yield 69.71 Ma, 62.49 Ma, and 66.55 Ma, defining the probable span of their magmatism from Late Cretaceous to Palaeogene. Based on their mineralogical and geochemical compositional diversity, the Khardung volcanic rocks are categorized as intermediate volcanic rocks (basaltic andesite-andesite) and felsic volcanic rocks (dacite-rhyolite). The intermediate volcanic rocks are marked by low SiO₂ (52.80-61.31 wt.%), enriched LREEs, and negative HFSEs (Nb, Ti, Zr) anomalies whereas,  felsic volcanic rocks are characterized by high SiO₂ (64.52-79.19 wt.%), pronounced negative Eu anomalies, enriched LREE and concave-downward HREE’s and negative HFSE’s (Nb, Ti) anomalies. Both the intermediate and felsic volcanic rocks exhibit quartz, sanidine, albite, bytownite, and diopside as their dominant mineral phases. Geochemical signatures indicate that the fractional crystallization and crustal contamination played a significant role in the evolution of the Khardung volcanic rocks and their geochemical diversity probably resulted from the partial melting of the common primary source, which had been metasomatized by variable contributions of fluids released from down going Neo-Tethyan oceanic crust. Thus, the Khardung volcanic rocks could be considered as a product of mature stage of arc magmatism during the subduction of the Neo-Tethyan oceanic crust, which occurred during Early Cretaceous to Palaeogene, prior to the main collision between the Indian and Asian plates.

How to cite: Lakhan Singh, N. and Krishnakanta Singh, A.: Geochemistry and uranium-lead isotopic ages of volcanic rocks associated with Ladakh batholith, western Himalaya: Implications for petrogenesis and tectonic evolution, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-113, https://doi.org/10.5194/egusphere-egu2020-113, 2020.

Oldoinyo Lengai is the only active carbonatite volcano within the East African Rift Valley in northern Tanzania. The volcano is dominated by peralkaline silicate rocks with natrocarbonatites. This study presents new mineralogical and geochemical data, including Sr–Nd–Pb–Hf–Mg isotopic compositions, for volcanic rocks at Oldoinyo Lengai and lavas from the nearby Gregory Rift Valley. The samples analyzed in this study include olivine melilitite, melanephelinite, wollastonite nephelinite, and phonolite. The olivine melilitites and melanephelinites have highly fractionated REE patterns with (La/Yb)_N values of 26.4–64.9, suggesting that they formed from magmas generated by low-degree (up to ~7%) of partial melting within the garnet stability field. The wollastonite nephelinites have much higher (La/Sm)_N values but lower (Sm/Yb)_N values relative to typical OIB, with flat HREE patterns [(La/Yb)_N = ~22]. The phonolites have elevated REE abundances but with patterns intermediate between the other two sample groups [(La/Yb)_N = ~41]. All samples have primitive-mantle-normalized incompatible element patterns that are characterized by negative K and Rb anomalies but no significant Eu anomalies. They also have elevated Yb contents relative to the compositions of modeled garnet peridotite-derived melts, suggesting that they were derived from a sublithospheric source containing enriched HIMU-like recycled oceanic crustal material. However, the wollastonite nephelinites have significantly positive Ba, U, Sr, and Pb anomalies similar to those found within the Oldoinyo Lengai natrocarbonatites. The wollastonite nephelinites might have been sourced from a region of sub-continental lithospheric mantle (SCLM) that was previously metasomatized by interaction with carbonatite melts. The phonolites in the study area have also weakly positive Pb and Sr anomalies indicative of some interaction with the SCLM. All samples have d²⁶Mg values (–0.39‰ ± 0.07‰) lighter than the composition of normal mantle material (–0.25‰ ± 0.04‰). In addition, a negative correlation between d²⁶Mg values and MgO concentrations suggests derivation from a source region containing recycled carbonate. The samples from the study area define a mixing array between HIMU- and EM1-type OIB in Sr–Nd and Pb–Pb isotopic correlation diagrams, and have pronounced Nd–Hf isotopic decoupling, plotting below the mantle regression line in Nd–Hf isotopic space. The negative deviation from the Nd–Hf isotopic mantle array and the presence of an EM1-type mantle component in the Sr–Nd isotopic compositions of the Oldoinyo Lengai volcanic rocks can be generated by recycling of E-MORB-type oceanic crustal material with an age of 1.5–1.0 Ga.

How to cite: Choi, S. H., Jung, S. G., and Ji, K. H.: Sr-Nd-Pb-Hf-Mg isotope geochemistry of volcanic rocks from Oldoinyo Lengai, Tanzania, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-192, https://doi.org/10.5194/egusphere-egu2020-192, 2020.

The Fröderyd Group constitutes a deformed volcanic sequence, which together with the 1834 Ma Bäckaby tonalites occurs as a xenolith, within the 1793-1769 Ma TIB 1b unit of the Transscandinavian Igneous Belt (TIB) in southern Sweden. The Bäckaby tonalites, together with coarse-grained clastic metasedimentary sequences of the Vetlanda Group, belong to the Oskarshamn-Jönköping Belt (OJB; Mansfeld et al., 1996). In turn, the Fröderyd Group was considered to be an older, probably Svecofennian, unit by Sundblad et al. (1997).

The Fröderyd Group is composed of ca. 80% mafic and ca. 20% felsic volcanic rocks, with subordinate carbonate units. Mafic rocks are represented by tholeiitic basalts and spilitized pillow lavas with MORB affinity.

In this study, a sample from a metamorphosed rhyolite, belonging to the Fröderyd Group, was dated at 1849.5±9.8 Ga U-Pb zircon age (LA-ICPMS). This age is significantly younger than the Svecofennian crust, which was formed from 1.92 to 1.88 Ga. Instead, it is coeval with the oldest TIB granitoid generation (TIB 0), which intruded into the southwestern margin of the Svecofennian Domain, but the Fröderyd Group is still the oldest crustal component southwest of the Svecofennian Domain.

Geochronological, petrographical studies and field observations have shown that the southern margin of the Svecofennian Domain was affected by ductile deformation shortly after the intrusion of the 1.85 Ga TIB granites (Stephens and Andersson, 2005). This took place during an intra- or back-arc rifting above a subduction boundary in a retreating mode and caused formation of augen gneisses and emplacement of 1847 Ga dykes into the TIB 0 granitoids. Rifting was followed by a collision of the rifted slab with the Svecofennian crust which is evidenced from emplacement of pegmatitic leucosomes during 1.83-1.82 Ga into the 1.85 Ga orthogneisses.

It is interpreted, that the Fröderyd Group was formed within an oceanic rifting environment, collided with the rifted Svecofennian slab and later amalgamated onto the Svecofennian Domain. The proposed geological evolution includes two deformation events during the period of ca. 1.85-1.82 Ga, which is in accordance with Röshoff (1975). Furthermore, it is evident that the Fröderyd Group was formed as a separate unit outside the Svecofennian Domain, although they have a common geological history.      

References

Mansfeld, J., 1996. Geological, geochemical and geochronological evidence for a new Palaeoproterozoic terrane in southeastern Sweden. Precambrian Res. 77, 91–103.

Röshoff, K., 1975. Some aspects of the Precambrian in south-eastern Sweden in the light of a detailed geological study of the Lake Nömmen area. Geologiska Föreningens i Stockholm Förhandlingar 97, 368–378.

Stephens, M.B. and Andersson, J., 2015. Migmatization related to mafic underplating and intra- or back-arc spreading above a subduction boundary in a 2.0–1.8 Ga accretionary orogen. Sweden. Precambrian Res. 264, 235–257.

Sundblad, K., Mansfeld, J. and Särkinen, M., 1997. Palaeoproterozoic rifting and formation of sulphide deposits along the southwestern margin of the Svecofennian Domain, southern Sweden. Precambrian Res. 182, 1–12.

How to cite: Salin, E., Sundblad, K., Lahaye, Y., and Woodard, J.: Recognition of a 1.85 Ga oceanic rifting environment in southeastern Sweden, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-424, https://doi.org/10.5194/egusphere-egu2020-424, 2020.

The Mesozoic granitic magmatism in Haliheiba is poorly understood because of lacking systematic data. Hence, this paper presents petrological observations, zircon U–Pb ages, geochemistry and Hf isotopes for these rocks. These rocks comprise granidiorite and quartz monzonite. Zircon LA-ICP-MS U–Pb dating yields emplacement ages of 247.6 ± 1.1 Ma and 247.0 ± 1.5 Ma for granidiorite and quartz monzonite, respectively. Geochemically, the granidiorite has SiO₂ contents of 65.86–67.37 wt% and alkali concentrations of 7.97–8.44 wt%; the quartz monzonite has SiO₂ contents of 66.95–67.28 wt% and alkali concentrations of 8.52–8.63 wt%, which belong to calc-alkaline series and are metaluminous rocks. These granitoids are enriched in light rare earth elements (LREEs) with (La/Yb)_N values from 5.27 to 12.09 and have slightly to moderately negative Eu anomalies with δEu values from 0.53 to 0.78 in the chondrite-normalized REE diagram. Furthermore, these granitoids are relatively enriched Rb, U, Th, K, and Pb and slightly depleted in Nb, Ta, Ba, Ti, and P in the primitive mantle-normalized spider diagram. The above geochemical signatures reveal that these granites have I-type affinity. Zircon Hf isotope data show that these granitoids possess high positive εHf(t) values from +8.9 to +14.9 and fairly young Hf model ages from 305 to 620 Ma, indicating that they are mainly derived from partial melting of juvenile crustal components. Combined with regional geology, our results indicate that the Triassic magmatism in Haliheiba most likely resulted from the subduction of the Paleo-Asian Ocean beneath the North China Craton. Our results together with regional isotopic data suggest that a significant crustal accretion event occurred during the Neoproterozoic to Paleozoic in the Great Xing’an Range.

How to cite: Wei, W. and Wu, X.: The Mesozoic granitic magmatism in Haliheiba: implications from geochronology, geochemistry and Hf isotopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1477, https://doi.org/10.5194/egusphere-egu2020-1477, 2020.

Accretionary orogens are characterized by voluminous juvenile components (recently derived from the mantle) and knowing the origin(s) of such components is vital for understanding crustal generation. Here we present field and petrological observations, along with mineral chemistry, zircon U–Pb age and Hf-O isotope data, and whole rock geochemical and Sr-Nd isotopic data for the c. 320 Ma Ulungur intrusive complex from the Central Asian Orogenic Belt. The complex consists of two different magmatic series: one is characterized by medium-K to high-K calc-alkaline gabbro to monzogranite; the other is defined by peralkaline aegirine-arfvedsonite granitoids. The calc-alkaline and peralkaline series granitoids have similar depleted mantle-like Sr-Nd-Hf isotopic compositions, but they have different zircon δ¹⁸O values: the calc-alkaline series have mantle-like δ¹⁸O values with mean compositions ranging from 5.2 ± 0.5‰ to 6.0 ± 0.9‰ (2SD), and the peralkaline granitoids have low δ¹⁸O values ranging from 3.3 ± 0.5‰ to 3.9 ± 0.4‰ (2SD). The calc-alkaline series were derived from a hydrous sub-arc mantle wedge, based on the isotope and geochemical compositions, under garnet peridotite facies conditions. This study suggests that the magmas underwent substantial differentiation, ranging from high pressure crystallization of ultramafic cumulates in the lower crust to lower pressure crystallization dominated by amphibole, plagioclase and minor biotite in the upper crust. The peralkaline series rocks are characterized by δ¹⁸O values lower than the mantle and enrichment of high field strength elements (HFSEs) and heavy rare earth elements (HREEs). They likely originated from melting of preexisting hydrothermally altered residual oceanic crust in the lower crust of the Junggar intra-oceanic arc. Early crystallization of clinopyroxene and amphibole was inhibited owing to their low melting temperature, leading to HFSEs and HREEs enrichment in residual peralkaline melts during crystallization of a feldspar-dominated mineral assemblage. Thus, the calc-alkaline and peralkaline series represent episodes of crust generation and reworking, respectively, demonstrating that the juvenile isotopic signature in accretionary orogens can be derived from diverse source rocks. Our results show that reworking of residual oceanic crust also plays an important role in continental crust formation for accretionary orogens, which has not previously been widely recognized.

How to cite: Tang, G.-J., Wang, Q., Wyman, D., Dan, W., Ma, L., Zhang, H.-X., and Zhao, Z.-H.: Petrogenesis of the Ulungur Intrusive Complex, NW China, and Implications for Crustal Generation and Reworking in Accretionary Orogens , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6679, https://doi.org/10.5194/egusphere-egu2020-6679, 2020.

A novel way to investigate the petrogenesis of ancient poly-metamorphosed terranes is to use zircon as a vessel and study protected mineral inclusions which are sensitive to melt evolution such as apatite. Recent contributions have shown that zircon-hosted apatite inclusions of unmetamorphosed granitoids can provide valuable petrogenetic information about a given pluton and, in turn, represent a way to circumvent effects of metamorphism. Yet, the impact of metamorphism on apatite inclusion has never been studied in detail. To address the issue of chemical and isotopic preservation of primary signals in apatite crystals both in the matrix and armored within zircons, we have studied apatite crystals from four 3.6-4.0 Ga orthogneisses of TTG affinity from the Acasta Gneiss Complex (Canada). Our results demonstrate that U-Th-Pb isotope systematics in matrix apatite crystals are reset at the time of the Wopmay orogen (1.8-1.7 Ga) whereas primary REE signatures were preserved in many crystals. On the contrary, zircon-hosted apatite inclusions all preserved primary REE signatures despite U-Th-Pb isotope systematics giving ages between 1.7 and 4.0 Ga. We interpret the variable resetting of these ages as a consequence of radiation damage accumulation in zircon lattice. Only the most pristine zircon has an apatite inclusion with a concordant age consistent with the magmatic age of the zircon (4.0 Ga). In addition, our results show that apatite crystals from TTG have distinct REE composition from post-Archean granitoids apatites, and that even apatites with reset ages preserved some of the chemical signatures characterizing TTG compositions (e.g. HREE). This capacity to retain primary information together with its discriminating power for granitoids makes apatite a very valuable tool for reconstructing the nature and evolution of ancient crustal rocks through the use of either detrital minerals or detrital-zircon hosting inclusions.

How to cite: Bruand, E., Antoine, C., Guitreau, M., and Devidal, J.-L.: Understanding preservation of primary signatures in apatite by comparing matrix and zircon-hosted crystals from the Eoarchean Acasta Gneiss Complex (Canada), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9848, https://doi.org/10.5194/egusphere-egu2020-9848, 2020.

Tonalite-trondhjemite-granodiorite gneisses (TTG) are the oldest litho-units of the Bundelkhand craton. The supracrustal rocks include variable deformed mafic volcanics and Banded Iron Formation. Magmatic zircons from the TTG’s yield an upper intercept of ~ 3590 Ma. The TTG’s gradually grades to a Na-feldspar rich A type porphyric granite towards the south. In this abstract, we report mineralogical, geochemical, and geochronological information of high silica- low Ca - high Na A-type granite from Bundelkhand craton.

In the TAS diagram, the studied samples plot in the field of granite and have a metaluminous affinity with high Ga/Al and Ce + Y + Nb + Zr values typical of A-type granites. In a primitive normalized multi-element spider diagram, the studied samples exhibit negative Nb, Ti, and P anomalies characteristics of a subduction zone setting. The chondrite normalized REE’s exhibit a strong fractionated pattern with negative Eu anomaly; the LREE are enriched and the HREE depleted with moderate to high (La/Yb)_CN ratios ranging from 11.12 to 26.24 ppm. The studied samples have plagioclase compositions that vary from X_Ab = 0.980-0.997 and chlorite compositions varying from X_Mg = 0.309-0.469.

Phase equilibria modeling yield an emplacement temperature of 700-750^OC, at 1.0 GPa. Most of the zircon grains are prismatic with visible cores and rims in optical examinations. In a U-Pb concordia diagram, the grains yield an upper intercept of 2536.6 ± 1.8 Ma. The geochemical and geochronological data taken together, indicate the Na-rich A-type granite generated by the high temperature and high-pressure partial melting of Archaean supracrustal rocks.

How to cite: Raza, M. B., Corfu, F., and Nasipuri, P.: Petrogenesis and Geochronology of the late-Archean Na-rich A Type granite from the Bundelkhand Craton, India: Implication for tectonic and crustal evolution, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16436, https://doi.org/10.5194/egusphere-egu2020-16436, 2020.

Early Mesozoic magmatism in Indochina and its vicinities in Sundaland (SE Asia) has been usually ascribed to be in connection with one of three approximately coeval tectonic regimes: 1) the Indochina-Sibumasu amalgamation leading to the closure of the Paleotethys during the Late Paleozoic – Early Mesozoic forming the Thai-Malaysia tin-bearing granite belt, 2) the Indochina-South China amalgamation along the northern boundary of Indochina closing another branch of the Paleotethys during Late Paleozoic – Triassic times, and 3) the early stage of an active margin with subduction of the Paleo-Pacific plate during Triassic-Jurassic times.

Scattered granitic plutons (185–210 Ma) located in southern Cambodia and some islands in southernmost Vietnam are distributed along the N-S Rach Gia-Nam Can fault which is a large-scale fault active during the Early Mesozoic. The studied rocks can be distinguished based on petrological features: weakly foliated biotite-rich granite (Hon Khoai Island, SW Vietnam), biotite-tourmaline-bearing granite (Hon Da Bac Island, SW Vietnam), and coarse-grained biotite granite (Tamao, SE Cambodia). The Honkhoai granites are a range of dark to light coloured granites due to a variation in biotite content and display a foliation. They usually contain amphibole, ilmenite, and monazite. The Hondabac granites comprise dark-colored granodiorites and granites with biotite, tourmaline, ilmenite, apatite, fluorite, epidote, and subordinate titanite. The Tamao granites are mainly composed of biotite aggregates with sporadic muscovite and accessory phases such as ilmenite, apatite, and fluorite.

Zircon U-Pb ages yield 189 ± 1 to 206 ± 2 Ma for the Honkhoai rocks, 192 ± 1 to 202 ± 1 Ma for the Hondabac rocks, and 189 ± 2 Ma for the Tamao rocks. Apparently, these Late Triassic - Early Jurassic granitoids are chronologically consistent with all three tectonic events. However, geographical and geochemical arguments favor a connection to the Thai-Malaysia tin-bearing granites. Similarities include high silica content and predominantly high-K to calc-alkaline affinities. Trace element composition is characterized by enrichments in Cs, Rb, Th, U, and Pb, and depletion in Ba, Sr, Nb, P, and Ti. All analyzed rock samples show (La/Yb)n values of 4.05–17.27 and negative Eu anomalies (Eu/Eu*=0.15–0.65). The whole-rock and biotite chemistry point to an arc-related tectonic setting for the Hondabac rock, while the Honkhoai and Tamao rocks are ambiguous in the tectonic regime but likely close to syn-collision and within-plate field, respectively. Geobarometry of the Honkhoai rocks using the Al-in-amphibole geobarometer yields crystallization pressure up to 3 kbar.

We conclude that the studied rocks formed during the closure of the Palaeotethys along the western boundary of the Indochina block, particularly similar to the Thai-Malaysia granite belt. Hence, the Sukhothai-Chantaburi Terrane may be extended southeastward as far as to the Hon Khoai Island (Southernmost Vietnam).

How to cite: Nong, A., Hauzenberger, C., Gallhofer, D., and Dinh, S.: Early Mesozoic granitoids from SW Vietnam and SE Cambodia – an example of the southeastern extension of the Southeast Asian granite belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16749, https://doi.org/10.5194/egusphere-egu2020-16749, 2020.

The Lower-Middle Jurassic volcanic rocks in the eastern Pontides were formed in a subduction zone under the extensional tectonic regime. These volcanic rocks were experienced seawater alteration during forming. They also were exposed to the burial metamorphism under the Cretaceous and Eocene aged formations. In addition, the Cretaceous and Eocene granitoids cut these volcanic rocks in some places and metamorphosed them. In this study, the mineralogical changes of the volcanic rocks they have experienced since their formation were examined.

Plagioclase (An_>42) + augite/diopside (En_38-52Wo_25-46 Fs_7-25) + Fe-Ti oxides (Fe⁺³/(Fe⁺³+Fe⁺²) > 0.80) ± magnesiohornblende (Mg/(Mg+Fe⁺²) > 0.92) are the main rock-forming minerals in these volcanic rocks. Mineralogical traces of seawater alteration are mostly masked by subsequent geological events. However, Na-enrichment of the plagioclases, increased ⁸⁷/⁸⁶Sr_(i) isotope ratios (0.70462 to 0.70611) and some clay minerals, laumontit,  analsime minerals, which are observed in the XRD peaks of some samples, refer to the alteration of the seawater. The pumpelyite (Fe⁺²/Fe⁺²+Mg = 0.60-0.90), chlorite (Fe_total/Fe⁺² + Mg = 0.15-0.95), sphene, calcite, dolomite and secondary quartz minerals were formed during burial metamorphism. The Fe-Ti oxides reached the chemical re-equilibrium under the new P-T conditions (magnetite Fe⁺³/(Fe⁺³+Fe⁺²) = 0.40-0.62; ilmenite Fe⁺³/(Fe⁺³+Fe⁺²) = 0.01-0.20). Epidote (Fe⁺²/(Fe⁺²+Mg) = 0.75-0.95) accompany the mineral paragenesis in some areas affected by Upper Cretaceous and Eocene granitoids.

Temperature estimations using the chlorite geothermometer and the phase relationships on the P-T diagrams show that the volcanics were heated up above 200°C in the buried areas where the granitoids were not effective. The temperatures were above 250°C in the areas where the magmatic rocks were effective. Taking into consideration the thickness of the formations that overlie the Jurassic volcanics, it can be suggested that the pressure affecting the Jurassic volcanics reached up to 1.5 kilobars.

Acknowledgement

This work was financially supported by Scientific and Research Projects Unit of Karadeniz Technical University with grant # 8920.

How to cite: Bak, T., Şen, C., Aydin, F., and Uysal, İ.: Low-Temperature-Low-Pressure Mineral Paragenesis of the Lower-Middle Jurassic Volcanics in the Eastern Pontides, NE Turkey, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18312, https://doi.org/10.5194/egusphere-egu2020-18312, 2020.

Based on the volcanostratigraphic studies, zircon U-Pb dating and geochemical data, the Late Cretaceous volcanic rocks (LCVs) from the Artvin region in the eastern Sakarya zone (NE Turkey) consist of mafic/basaltic (S1-Çatak and S2-Çağlayan) and felsic/acidic (S1-Kızılkaya and S2-Tirebolu) rock types that occurred in two successive stages: (i) first stage (S1: Turonian to Early Santonian) and (ii) second stage (S2: Late Santonian to Campanian). In both stages, the basaltic rocks contain generally calcic plagioclase and lesser augite crystals, whereas the acidic samples commonly contain quartz, sodic plagioclase and K-sanidine phenocrysts. Data from clinopyroexene thermobarometry point to the S2-Çağlayan basaltic rocks having crystallised at higher temperatures and under deeper crustal conditions (T = 1128 ± 15 ^oC, P = 6.5 ± 0.7 kbar and D = 19.5 ± 2.1 km) than those of the S1-Çatak rocks (T = 1073 ± 11 ^oC, P = 2.2 ± 1.0 kbar, D = 6.6 ± 3.0 km).

The LCVs show a wide compositional spectrum, ranging from tholeiite to calc-alkaline/shoshonite and are typically represented by a geochemical composition resembling subduction-related arc rocks although the ⁸⁷Sr/⁸⁶Sr_(i) (0.7044–0.7071) and ɛNd_(i) values (-0.63 to +3.47) as well as ²⁰⁶Pb/²⁰⁴Pb_(i) (18.07 to 18.56), ²⁰⁷Pb/²⁰⁴Pb_(i) (15.57 to 15.62) and ²⁰⁸Pb/²⁰⁴Pb_(i) (37.12 to 38.55) ratios show very limited variation. The average δ¹⁸O isotope values of the S1-Kızılkaya (5.3 ± 0.5‰) and S2-Tirebolu (4.9 ± 0.8‰) zircons are quite consistent with average mantle values (5.3 ± 0.3‰). The similar isotopic compositions of the studied mafic and felsic volcanic rocks, and the relatively high Mg# values (up to 0.4–0.51) of the felsic samples indicate a cogenetic origin. The parent magmas of the S1-Çatak and S2-Çağlayan mafic volcanic rocks were derived from underplated basaltic melts that originated by partial melting of metasomatised spinel lherzolite and spinel-garnet lherzolite, respectively. It is proposed that the compositions of the S1-Kızılkaya (mainly dacitic) and S2-Tirebolu (rhyolitic to trachytic) felsic rocks were particularly controlled by metasomatised mantle–crust interaction and MASH zone plus shallow crustal fractionation processes.

Our data, together with data from previous studies, suggest that the S1- and S2-mafic and felsic rock types of the LCVs (~95–75 Ma) are the products of two-stage volcanic event that took place during the northward subduction of the northern Neotethys Ocean.

Acknowledgement

This study was financially supported by Scientific and Technological Research Council of Turkey (TUBITAK) with grant# 112Y365.

How to cite: Oğuz Saka, S., Aydin, F., Şen, C., Dokuz, A., Aiglsperger, T., Uysal, İ., Kandemir, R., Karsli, O., Sarı, B., and Başer, R.: Petrological and geodynamic evolution of the Late Cretaceous subduction-related volcanism in the eastern Sakarya Zone, NE Turkey, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21241, https://doi.org/10.5194/egusphere-egu2020-21241, 2020.

Although the presence of Latest Cretaceous intrusives (~70 Ma) and the early Eocene adakitic magmatic rocks (~57-50 Ma) in the eastern Sakarya Zone are well-known, the early Eocene non-adakitic rocks are very limited and have not been studied in terms of tectono-magmatic evolution. We described a small outcrop of non-adakitic quartz diorite porphyry in Kov area of the Gümüşhane from NE Turkey of which the genesis is significant in evaluating the syn- to post-collision-related magmatism. The LA-ICP-MS zircon U-Pb dating reveal that the Kov quartz diorite porphyries formed at ca. 50 Ma, coeval with adakitic rocks, ~20 Ma later than the slab roll-back-related intrusive rocks. The Kov porphyries are calc-alkaline in composition and enriched in large ion lithophile elements (LILEs), light rare earth elements (LREEs) and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, Ti), with significant negative anomalies at Nb, Ta, and Ti but positive anomalies at Th, U, and Pb. Isotopic compositions of the samples show limited range of variation and slight enrichment of ⁸⁷Sr/⁸⁶Sr_(i) (0.70489 to 0.70555), e_Nd(i) (-1.4 to -1.2) with T_DM  of 1.11 to 1.61 Ga. Pb isotopic ratios of the samples point to an enriched mantle source. They probably were crystallized from the melt that originated by low-degree partial melting (~1-2%) of an EMII-type spinel-facies subcontinental lithospheric mantle (SCLM), followed by the fractionation of clinopyroxene with insignificant crustal assimilation. The SCLM was metasomatically enriched and the metasomatic agent was likely H₂O-rich fluids rather than sediments released from subducting oceanic crust during the Late Cretaceous closure of the Neotethyan oceanic lithosphere.

In conjunction with the geological background and previous data, we envisage that generation of the Kov porphyries is resulted from a slab break-off event that caused ascending or infiltration of hot asthenosphere triggering mantle melting. Such sporadic occurrences of the porphyries, with coeval adakitic rocks in the Sakarya Zone are likely associated with the onset of extensional tectonics due to the earlier stage of slab break-off along the region during early Eocene period.

Acknowledgement

This work was financially supported by Scientific and Technological Research Council of Turkey (TUBITAK) with grant #108Y200.

How to cite: Karsli, O., Uysal, İ., Aydin, F., Dokuz, A., Şengün, F., Kandemir, R., Oğuz Saka, S., and Santos, J.: Petrogenesis and geodynamic significance of the early Eocene quartz diorite porphyries from the eastern Sakarya Zone, NE Turkey, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21346, https://doi.org/10.5194/egusphere-egu2020-21346, 2020.

     The tectonic evolution and affinity of the Alxa Block has long been controversial. The NW-SE trending Longshoushan Belt is in the southwestern margin of the Alxa Block, separated the Qilian Block. In this study, we present zircon U-Pb and Hf-isotope data of the middle and eastern Longshoushan, which could constrain the provenance and formation age of the Longshoushan Belt, and further constrain the tectonic evolution and affinity of the Alxa Block. The U-Pb ages of the detrital zircons from the amphibolite-facies metamorphosed volcanic-sedimentary rocks of the middle Longshoushan range from 3006 to 1981 Ma (peak at 2010 Ma), which were consistent with the Alxa Block and the western North China Craton, indicating that the middle Longshoushan was deposited in the Palaeoproterozoic, not in the Archean, and had tectonic affinity with the Alxa Block and the western North China Carton. Combined with the identical crustal growth events at 2.4-2.5 Ga of the middle Longshoushan, the Alxa Block and the western North China Craton, the Alxa Block was an integrated part of the Western Block of the North China Craton. The U-Pb ages of the detrital zircons from the greenschist-facies metamorphosed volcanic-sedimentary rocks of the eastern Longshoushan range from 3389 to 529 Ma (peak at 2.5 Ga and 1.0 Ga), which were highly consistent with Hexi Corridor, indicating that the eastern Longshoushan was deposited in the Cambrian, and had an affinity with the Hexi Corridor. In the Early Palaeozoic, the North Qilian Ocean subducted the Alxa Block and formed a typical trench-arc-basin system. With the closure of the North Qilian Ocean, the Central Qilian Block collided with the Alxa Block, formed the eastern Longshoushan, which was a foreland basin in the Hexi Corridor.

How to cite: Liu, J., Yin, C., Zhang, J., Qian, J., Xu, K., Wu, S., and Xu, N.: Detrital zircon U-Pb and Hf isotopes study of the Longshoushan Belt in the southwestern margin of the Alxa Block: Constraints on the tectonic evolution and affinity of the Alxa Block, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22176, https://doi.org/10.5194/egusphere-egu2020-22176, 2020.