Mineralogy and Geochemistry of mafic to hybrid microgranular enclaves and felsic host of Ladakh batholith, Northwest Himalaya: Evidence of multistage complex magmatic processes

Abstract

HIMALAYAN JOURNAL OF SCIENCES | VOL. 5 | ISSUE 7 (SPECIAL ISSUE) | 2008 130 Felsic magmatisms in the north of Indus-Tsangpo-Suture Zone, referred herein Ladakh granitoids (LG), have been characterized dominantly as calc-alkaline, magnetite to ilmenite series (gradually changing from NW to SE) granitoids derived from partial melting of heterogenous protoliths in subduction environment. Field relation, petrography, mineralogy and geochemistry of ME and felsic host LG have been carried out along various transects covering Northwest (Dras-KargilSilmo-Batalik-Achina Thang), Central (Leh-Ganglas-S. PulluKhardungLa-N. Pullu-Khardung), Southeast (Himiya-LitseUpshi-Karu-Sakti-Zingral-ChangLa) parts of Ladakh batholith in order to understand the physical and chemical processes of mafic and felsic magma interaction in plutonic condition. The LG can be broadly classified into coarse-grained LG with abundant mafics (hbl-bt), medium-grained LG with low content of mafics, and fine-grained leucocratic LG with very low amount of mafics. Mesocratic to melanocratic, fine to medium grained and porphyritic (xenocrystic) hybrid microgranular enclaves (ME) are found hosted mostly in medium to coarsegrained LG. Double enclaves (one mafic ME into other porphyritic ME) and syn-plutonic disrupted mafic dykes are also hosted in K-feldspar megacrysts bearing LG in the Northwest part of batholith, which record evidence of multiple mafic to hybrid magma injection and thermal rejuvenation of partly crystalline LG. ME are absent or rare in the leucocratic variety of LG. The ME are rounded to elongate commonly having sharp, crenulate, and occasionally diffuse contacts with felsic host, and size varies from a few cm to metres across but cm-sized ME (d<30 cm) are common, which strongly suggest that several pulses of crystal-charged mafic and felsic magmas, coexisted, hybridized, co-mingled into plutonic setting. Most hybridized parts (active mixing region) of LG do not contain ME whereas isolated mixing regions of LG contain variable sizes of mingled and streched (sine flowage) ME. The ME (diorite, quartzdiorite) and felsic host LG (granodiorite, monzogranite) bear common mineral asemblages (hbl-bt-pl-kfs-qtz-ap-zrnmt ±ilm) but differ in their mineral proportions. The ME lack cumulate-like texture and are fine to medium grained, and therefore oppose their co-genetic link with LG. Presence of patchy zoned (spongy, cellular) plagioclase xenocrysts, quartz ocelli and apatite needles in porphyritic ME strongly indicate mingling and undercooling of hybridized ME globules into cooler felsic host LG. Grain size differences, crystal index, and size of ME among the ME population, except to those of porphyritic ones, correlate well with degree of undercooling of ME. Disaggregated large ME into several smaller ones Mineralogy and Geochemistry of mafic to hybrid microgranular enclaves and felsic host of Ladakh batholith, Northwest Himalaya: Evidence of multistage complex magmatic processes