Migmatite

Abstract: Preface The historical perspective - an ever changing emphasis The categories of granitic rocks - the search for a genetic typology Granite as a chemical system - the experimental impact The physical nature of granitic magmas - a case of missing information The evolution of the granitic texture - a continuum of crystal growth Differentiation in granitic magmas: zoning as an example of multifactinal processes at work The volcano-plutonic interface - not Read's hiatus The evidence for restite - unmixing as an alternative hypothesis The mingling and mixing of granite with basalt - a third term in a multiple hypothesis Appinites, diatremes and granodiorites - the interaction of wet basalt with granite Controls of upwelling and emplacement - the response of the envelope: balloons, pistons and reality On the rates of emplacement, crystallization and cooling Plagiogranite and Ferrogranophyre: extreme differentiation in contrasted situations Cordilleran-type batholiths - magmatism and crust formation at a plate edge Intraplate rift-related magmatism - mainly the A-type, alkali feldspar granites Migmatites - are they a source of granitic plutons? The waning stages - the role of volatiles in the genesis of pegmatites and metal ores The sources of granitic magmas in their various global tectonic niches A kind of conclusion - a search for order among multifactorial processes and multifarious interactions Bibliography Index

Abstract: The petrology and timing of crustal melting has been investigated in the migmatites of the Higher Hima- layan Crystalline (HHC) exposed in Sikkim, India. The metapelites underwent pervasive partial melting through hydrous as well as dehydration melting reactions involving muscovite and biotite to produce a main assemblage of quartz, K-feldspar, plagioclase, biotite, garnet ± sillima- nite. Peak metamorphic conditions were 8-9 kbar and *800 C. Monazite and zircon crystals in several migmatites collected along a N-S transect show multiple growth domains. The domains were analyzed by micro- beam techniques for age (SHRIMP) and trace element composition (LA-ICP-MS) to relate ages to conditions of formation. Monazite preserves the best record of meta- morphism with domains that have different zoning pattern, composition and age. Zircon was generally less reactive than monazite, with metamorphic growth zones preserved in only a few samples. The growth of accessory minerals in the presence of melt was episodic in the interval between 31 and 17 Ma, but widespread and diachronous across samples. Systematic variations in the chemical composition of the dated mineral zones (HREE content and negative Eu anomaly) are related to the variation in garnet and K-feldspar abundances, respectively, and thus to meta- morphic reactions and P-T stages. In turn, this allows prograde versus decompressional and retrograde melt production to be timed. A hierarchy of timescales charac- terizes melting which occurred over a period of *15 Ma (31-17 Ma): a given block within this region traversed the field of melting in 5-7 Ma, whereas individual melting reactions lasted for time durations below, or approaching, the resolution of microbeam dating techniques (*0.6 Ma). An older *36 Ma high-grade event is recorded in an al- locthonous relict related to mafic lenses. We identify two sections of the HHC in Sikkim that traversed similar P-T conditions at different times, separated by a tectonic dis- continuity. The higher structural levels reached melting and peak conditions later (*26-23 Ma) than the lower structural levels (*31-27 Ma). Diachronicity across the HHC cannot be reconciled with channel flow models in their simplest form, as it requires two similar high-grade sections to move independently during collision.