Summary of structurally-controlled melt accumulation

Migmatite continued to be dominant in the upper parts of the HHC, where the Bhapkund Formation mainly contains these important rocks. Critical field observations highlight the following five stages of structurally-controlled melting and melt occurrences, Me1 to Me5 (Jain et al., 2013).

First Melt Stage (Me1): The earliest pervasive stage of melt occurrence is generally associated with the D1 deformation in the Bhapkund Formation. This stage has typically produced layered stromatic migmatite, parallel to the first visible and oldest foliation Sm within the country rocks (Figs. 20B; 30A). New leucosome imperceptibly grades and permeates the mesocratic grey paleosome of the host gneiss/schist, having large decimeter thick foliated leucogranite, as well. The most obvious feature of Me1 generated melt is the presence of a strong foliation within leucosome and parallelism with the strong tectonic foliation (Fig. 30A).

Second melt stage (Me2): Stromatic migmatite has undergone very strong SW-verging asymmetrical and nearly-isoclinal folding during the D2 event, which has transposed the Sm foliation into new S2 foliation, when highly-strained limbs are attenuated and thrust towards SW along ductile shear zones. Many of these limbs and shear zones are now occupied by very thin smear of new leucogranite during the Me2 stage of melt generation and accumulation (Fig. 31A). As the leucosomes of the Me1 stage becomes thicker in the fold hinges, considerable flowage took place from the attenuated limbs into hinges. Possibly, the melt has also flown along the thrusts, bounding the limbs during this stage (Fig. 31A).

Third Melt Stage (Me3): The uppermost amphibolite-facies and sillimanite bearing gneiss exhibits discrete normal-sense shear bands to the south of Malari where earlier foliation Sm has developed SW- and NE- dipping ductile shear bands, now occupied by thin leucosomes (Figs. 21; 31B). These bands die out within the main foliation and thus, define the extensional flanking structures. Both the conjugate set reveal an episode of layer-parallel extension, possibly related to the movements along the STDS.

In addition, a late stage layer-parallel extension has produced extensional foliation boudins whose necks are now occupied by structureless leucosome (Fig. 21). The Sm foliation within the melanosome swings into the neck both ways and is deformed around neck of the boudins where newly-developed leucosome occupies sites of decompressed necks. In contrast to the leucosome of the shear bands, this new leucosome within the necks remains structureless and appears to have developed late during decompressional melting of the HHC.

Fourth Melt Stage (Me4): This stage of melt formation basically incorporates veins and veinlets across the earlier fabric (D1) and is developed mainly on the Me1 stage stromatite (Figs. 31C, D). Tourmaline-rich ataxial leucocratic pegmatite extensional veins are structureless where quartz, feldspar and tourmaline fill in the cavities. Dilatant veins are seen intruding Me1 layers and surround them completely with breccia-like agmatitic appearance.

Fifth Melt Stage (Me5): This is the last stage of melt formation where melt has accumulated as structureless net-like patches on the foliation Sm, which is totally destroyed in the region now occupied by these patches (Fig. 23).