Fluid-Assisted Deformation: Rhomb Slip Preference in Quartz from Metasomatic Reaction Zones of a Mobile Belt in India

Quartz slip systems are conventionally linked with their corresponding temperatures of activation, but fluid can affect them as well; and how the presence of syn-deformational fluid affects the slips system activation remains poorly constrained. Quartz textures are result of integrated effects of P-T-fluid-deformation, making it challenging to isolate the individual contribution of any single factor. The occurrence of metasomatic reaction zones (MRZs) due to fluid-rock interactions at the boundary between country rock, i.e., a pelitic garnet-mica schist and meta-mafic dykes (Dyke-I and -II) in the Northern Singhbhum Mobile Belt (NSMB) of eastern India, provides an opportunity to address this problem. This geometrically well-constrained system, whose P-T-reaction history has been petrologically and geochemically characterized, allows us to isolate and examine how fluids affect the quartz microstructure at same P-T-deformation conditions. We investigated samples from MRZs, using bulk-rock geochemistry, mineral chemistry, thermodynamic modelling and electron backscatter diffraction analysis.

Geochemical-reaction-path models show that MRZs (amphibole-epidote-plagioclase-quartz and chlorite) assemblage formed by Na-metasomatism at 2–3 kbar and 300–500°C after a post-peak condition (6–8.5 kbar and 550–600°C) of NSMB. The saline fluids reacted with the dykes, i.e., the source and then reacted with the pelite, facilitating the element-mass exchange between them. Our study covers two different scenarios, a fluid-abundant MRZs near Dyke-I (zone 1) and another fluid-limited near Dyke-II (zone 2). Zone 1 exhibits a plagioclase-quartz dominated polygonal mosaic matrix with complete removal of muscovite and garnet. The matrix is characterized by pervasive brown-colored anastomosing fluid networks along grain boundaries, fractures, and cleavages. Healed fractures containing Fe-oxide and fluid inclusion trails are abundant, and small epidote grains occur at grain boundaries and triple junctions. Zone 2 is more quartz-dominated with granoblastic texture subhedral grains showing straight to curved boundaries. Relict biotite and garnet are preserved. The matrix quartz shows isolated microfractures and trans-crystal fluid inclusion trails but lacks the extensive interconnected fluid-network architecture of Zone 1.

The slip system of quartz transitions from the country rock towards the two MRZs. Deformation in quartz of the country pelitic schist, was accommodated mainly via activation of prism <a> and <c> slips. They record abundant presence of <2° kernel average misorientation (KAM). Quartz in zone 1 shows deformation accommodation via rhomb <a> slip and near-complete absence of <2° KAM. The intensity of rhomb <a> slip increases towards its dyke-contact. Whereas in zone 2, quartz shows dominant prism <a> slip and abundant <2° KAM, just like the country rock. Near its corresponding dyke contact of zone 2, the quartz shows polygonization, emergence of rhomb <a> slip, and reduction in <2° KAM due to a relatively higher proportion of fluid presence at the contact.

This study demonstrates how fluid-rock-interaction intensity can play significant role in quartz deformations and display a preferred slip system activity under the same prevailing P-T condition. We propose that under fluid-abundant conditions, the quartz polygonised and rhomb <a> slips are activated in zone 1 due to complex reaction creep and hydrolytic weakening resulting from fluid-rock interactions at the same P-T-deformation conditions.