Lamprophyres from Ospa ophiolite of the East Sayan (Russia)

In Eastern Sayan mountains (E Siberia), ophiolite complexes form three extended branches: 1 - Ilchir (MOR ophiolites), 2 - Ospa-Khara-Nur (suprasubduction zone (SSZ) and volcanic arc (VA) ophiolites) and 3 - Shishkhid-Yehe-Shignin (back-arc ophiolites).

Lamprophyre (L) dykes or mica peridotites (Shestopalov, 1938) were found in brecciation zone of ophiolites (dunites, harzburgites, serpentinites) of the Ospa-Khara-Nur peridotite complex. They form bodies to 1m thick, and vein-like fragments in intensively deformed and altered (serpentinized, tremolitized) ultramafic rocks.

Dark gray massive porphyric L correspond to the range between ultramafic (UML), alkaline (AL), and Ca alkaline lamprophyre (CAL), and lamproite lamprophyres (LL) according to (Rock, 1991) and show compositional range in MgO-CaO, - Al₂O₃, - Na₂O, - P₂O₅ diagrams. Lamprophyre rocks consist of feldspar, phlogopite, orto- and clinopyroxene, amphibole, with relics of olivine (Fo=45-50, rarely 22-30) and large (up to 1 cm) porphyric phlogopite. In more acid L of CAL type with prevailing hypersthene and fieldspars are associated by amphiboles metasomatic type (ferro-eckermannite, actinolite, tremolite) and rarely metamorphic glaucophane. Micas grains from phlogopite to biotite (0.2-1.7% and 2.1-2.8% TiO₂) are surrounded by sericite. Feldspar vary albite to anorthite, and rare grains of orthoclase and Ba-feldspar. Fluorine-apatite (Cl to 0.3%), ilmenite, rutile are common in L but zircon, monazite and Ce-La-epidote are rare. Mineral thermometry range from 1300^oC to 950^oC for LL then 850^oC -560^oC and low metamorphic stage.

TRE from L shows inclined REE with flat La-Sm, HFSE troughs but high LILE. The acid CL reveal Eu peak (Eu*=3,2; (La/Yb)_n=9). Spider and REE diagram reveal elevated HFSE, Sr, Pb the same high LILE closer to anorthosites and pegmatiod charnokites. This suggests that high extremely high temperature ML reacted with acid rocks and produced Ca-alkaline L type.

Age spectra were obtained for phlogopites from lamprophyres by ⁴⁰Ar/³⁹Ar step heating method. In sample VS-66-2, spectrum reveal intermediate plateaus of 3 stages (32%, 35%, 33%) of cumulative ³⁹Ar with ages 950 ± 6 and 976 ± 6 Ma, respectively. In the spectrum VS-57 a good plateau 902 ± 9 Ma is distinguished (79% of cumulative ³⁹Ar). Most discordant spectrum VS-52 reveals 4 stages of creation. Most likely the age of L formation is - 976 ± 6 Ma and corresponds to ocean stage. Most likely, and 902 ± 9 corresponds to the age of the intensive deformation later event in subduction zone. Further deformation suggests the complex tectonic-thermal history.

We suggest that late Proterozoic ophiolites which refer to oceanic stage of 1100 Ma were later incorporated to arc complex with the acid base. At 980 Ma they were subjected to plume event with the creation of UML due to reaction with crust and the they were hybridized with acid rocks to produce CAL. Late alteration produced series of secondary minerals. Thus the UML, and AL, and CAL give more information about the history of ophiolites of the Eastern Sayan.

This work supported by RFBR grants: No. 19-05-00764 and the Russian Ministry of Education and Science.

References:

1. Shestopalov M.F. // In: Gemstone workbook. V. 4. 1938. P.84-100.
2. Rock N.M.S. Lamprophyres. Springer Science+Business Media, LLC. 1991.