Polybaric and multistage fractionation of syn-collision mafic magma in continental arc: constraints from the Eocene mafic rocks in the Tengchong Block, southeastern extension of Tibet

Although the arc magmatism before collision has been considered as the main mechanism to the continental crustal growth and vertical geochemical fractionation for many years, the syn-collision magmatism related to the melting of accumulation in the base of arc could be an important contribution to crustal net growth and fractionation. Hence, the syn-collision magmatism could be an ideal object to research the continental crustal maturation and stratification. The arc magmatism could be controlled by the connecting magmatic reservoir in different depth and the experimental data show that the arc magma could be polybaric fractionation. However, the detail fractional phase in different level is not clear. Therefore, we selected the Early Eocene mafic rock series in the Tengchong Block, southwestern extension of Tibet, to reveal the detail magmatic evolution process. The rocks include hornblendite, hornblende (Hb) gabbro and diorite with different mineral assemblages, which is the syn-collision magmatism related to the Indian-Asian continental collision. These rocks have zircon ages of ca. 54Ma, and similar whole rock Sr-Nd and zircon Hf isotopes, indicating they are coeval and congenetic. In contrast to the isotopic composition, the major elements of the suits are variable, such as SiO₂ contents of 48.72-61.49 wt.%, MgO contents of 12.02-2.69wt.%. The clinopyroxene (cpx) is mainly enclosed in the hornblende in the samples and part of the Hb could be the products of the replace reaction associated with cpx and others could be direct crystallization from the magma. The crystallization parameters calculation results show that the clinopyroxenes have high pressures of 2.4-10.7kbar with average of 7.6kbar and temperatures of 1006-1208°C with average of 1154°C. The hornblende crystallized at the pressures of 2.2-7.8kbar with average of 3.8kbar, and temperatures of 776-875°C with average of 827°C. In addition, the plagioclases in the all samples have three types, including high An core, low An rim with overgrowth rim as type I, low An core, high An mantle low An rim with overgrowth rim as type II, low core with overgrowth rim as type III. The homogeneous in-situ Sr isotopes show the compositions variation from the core to rim could be resulted from the process of dissolution and reprecipitation during the batches recharging of homogeneous magma. Therefore it could conclude that the primary magma of the Eocene mafic rocks could be fractionated in the lowermost crust, and the major crystallization phase dominated by clinopyroxene and forming the pyroxenite as the base of the arc. Then the evolution mafic magma emplace and form a mafic reservoir in the middle crust according to the assembly of batches of magma and finally occurring the further fractionation that the hornblende-dominated accumulation forming the hornblendite and the hornblende and plagioclase accumulation forming the Hb-gabbro and diorite. This polybaric fractionation within the continental crust during syn-collision could lead to the melt transition from mafic to granitic and further strengthen the crustal maturation and stratification.

Supported by National Natural Science Foundation of China [Grant Nos. 42272052 and 41902046], Fundamental Research Funds [Grant No. 300102273102]