Greenschist-facies sub-ophiolitic metamorphic rocks of Andaman Islands,Burma-Java subduction complex

摘要

Sub-ophiolitic greenschist facies metamorphic rocks occur at the sole of ophiolite slices and as blocks in the melange zone beneath the Andaman ophiolite. These are represented by metabasics as actinolite schist to actinolite-chlorite schist and metasediments as garnetiferous quartzo-feldspathic mica-chlorite schist and piemontite quartzite to piemontite bearing quartz-muscovite-chlorite schist to muscovite-quartz-chlorite schist. Actinolite occurs along the schistosity and also as porphyroblasts. Syn to post-tectonic garnet shows no compositional zoning and represent almandine-spessartine solid solution (Alm_(44-17) Sps_(23-27). Grosi_(3-17), Pyr_(9-10)). The metabasics are enriched in L1LE and depleted in Zr and Y compared to N-MORB.rnThe lithological features suggest that residual heat was the main heat source for greenschist-facies meta-morphism. Top part of the subducting slab and overlying trench sediments were metamorphosed and dislocated by the close spaced thrusts in an accretionary prism setting. The field association indicates that metamorphism and the uplift of metamorphic rocks along with ophiolite slices were bracketed in between Cretaceous and Oligocene period. These processes were later than the Pre-Cretaceous emplacement of the phiolites of Sumatra and Java.