Abstract

Shear-hosted, sub-cm to 50-m-wide, Fe‑Ti-Zr-REE (FTZR)–rich melt bands occur exclusively at folded margins of the Bolangir anorthosite pluton (eastern India) transverse to margin-parallel foliation in anorthosite. The N- to NNW-trending FTZR melt bands are oriented at a low angle (<20°) to the inferred shear plane. In FTZRs, tiled grains and trains of magmatic pyroxene in dynamically recrystallized plagioclase aggregates indicate that crystal-suspended melt flow was synchronous with E-W shortening deformation that outlasted melt solidification. Mass balance–accommodated, mineral-melt calculations suggest the abundances of major element oxides, large ion lithophile elements (LILEs), high field strength elements (HFSEs), and rare earth elements (REEs) in anorthosite and complementary-to-anorthosite FTZR residual melts are best explained by >90% fractionation of anorthosites from crustally contaminated, high-Al gabbro (HAG) parent melt. At the high degree of crystal fractionation (crystal content higher than the particle-locking threshold), the FTZR melts, during last stages of crystallization, were locked within the plagioclase-dominated cumulus pile. The occurrence of FTZR melt bands exclusively at folded—as opposed to straight segments of—pluton margin is attributed to melt-enhanced strain localization in the pluton induced by the FTZR melts locked within anorthosite. Further melt-induced straining led to focusing of the FTZR melts along dilatant shear zones formed synchronously with the east-west shortening deformation; the variable widths of the melt bands correspond to the varying sizes of actively deforming magmatic plagioclase in anorthosite. The extraction and transport of locked-in, low-volume fraction melt documented here suggest long-duration, far-field stresses may segregate and transport large volumes of low-volume fraction melts in planetary interior over considerable distance.

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