Microstructural evidence for magma confluence and reusage of magma pathways: implications for magma hybridization, Karakoram Shear Zone in NW India

Abstract

In the Karakoram Shear Zone, Ladakh, NW India, Miocene leucogranitic dykes form an extensive, varied and complex network, linking an anatectic terrane exposed in the Pangong Range, with leucogranites of the Karakoram Batholith. Mineral paragenesis of the heterogeneous anatectic source rocks suggests melting has resulted from water influx into rocks at upper amphibolite facies conditions, and microstructures suggest anatexis was contemporaneous with shearing. The network is characterized by continuous and interconnected dykes, with only rare cross-cutting relationships, forming swarms and chaotic injection complexes where magmatic rocks cover up to 50% of the outcrop area. Despite this volume of magma, the system did not lose continuity, suggesting that it did not flow en masse and that the magma network was not all liquid simultaneously. Leucogranites in this network, including leucosomes in migmatites, carry an isotopic signature intermediate between the two main anatectic rocks in the source, suggesting efficient homogenization of the magmatic products. Here, we describe a number of microscopic features of these magmatic rocks which suggests that several pulses of magma used the same pathways giving rise to textural and chemical disequilibrium features. These include: (i) narrow, tortuous corridors of fine-grained minerals cutting across or lining the boundaries of larger grains, interpreted to be remnants of magma-filled cracks cutting across a pre-existing magmatic rock; (ii) corrosion of early formed grains at the contact with fine-grained material; (iii) compositional zoning of early formed plagioclase and K-feldspar grains and quartz overgrowths documented by cathodoluminescence imaging; (iv) incipient development of rapakivi and anti-rapakivi textures, and (iv) different crystallographic preferred orientation of early formed quartz and fine-grained quartz. Mapping of the fine-grained corridors interpreted to represent late melt channels reveal an interlinked network broadly following the S-C fabric defined by pre-existing magmatic grains. We conclude that early formed dykes provided a pathway exploited intermittently or continuously by new magma batches. New influxes of magma opened narrow channels and migrated through a microscopic network following predominantly grain boundaries along an S-C fabric related to syn-magmatic shearing. A mixed isotopic signature resulted not from the mixing of magmas, but from the micro-scale interaction between new magma batches and previously crystallized magmatic rocks, through local equilibration.

Synkinematic magmatism, heterogeneous deformation, and progressive strain localization in a strike‐slip shear zone: The case of the right‐lateral Karakorum fault

“…At a macroscopic scale, the close relationship of the dyke network with the structures resulting from right‐lateral shear lead [ Reichardt and Weinberg , 2012] to propose that magma migration was controlled by stresses related to right‐lateral transpression. At a microscopic scale, Hasalová et al [2011]showed that late melt channels follow two distinct orientations, parallel to the S‐C fabric resulting from right‐lateral shear in the KSZ, thus implying that magma migration was coeval with the deformation. It is worth to note that none of the Miocene intrusions shows structures that could suggest it has been emplaced with a dip significantly different from its present‐day geometry.…”

“…Two opposing views have been proposed. Some advocate for a main magmatic phase antecedent to the onset of deformation and few leucocratic dykes emplacement after that deformation [e.g., Phillips et al , 2004; Searle and Phillips , 2007], while other think that at least part of the magmatism was coeval to strike‐slip deformation [e.g., Hasalová et al , 2011; Leloup et al , 2011; Reichardt and Weinberg , 2012; Weinberg et al , 2009; Weinberg and Mark , 2008].…”

“…Other authors have proposed that melting and magma migration within the Pangong range was coeval to the right‐lateral shearing [ Hasalová et al , 2011; Reichardt and Weinberg , 2012; Weinberg et al , 2009; Weinberg and Mark , 2008]. In the Pangong Range, migmatitic magmas formed by local anatexis and migrated during folding into axial‐planar leucosomes [ Weinberg and Mark , 2008].…”

Role of strain localization and melt flow on exhumation of deeply subducted continental crust

“…Many have summarized criteria for recognition of former melt at grain scale in metamorphic rocks (e.g., Závada et al, 2007;Holness and Sawyer, 2008;Hasalová et al, 2008bHasalová et al, , 2011. Using these criteria we interpret the described interstitial films, and pools in the banded orthogneiss, migmatite, and granofels, as former crystallized melt.…”

“…The amount of crystallized melt is variable, being highest in migmatites and granofelses (>16 vol%; Table 2) compared to 5-10 vol% in the orthogneisses. However, it has been shown that the amount of crystallized new magmatic phases is not necessarily proportional to the amount of melt that passed through the rock, as the shear zones can be regarded as open systems (Hasalová et al, 2008b(Hasalová et al, , 2008c(Hasalová et al, , 2011Goncalves et al, 2012). Alternatively, large interstitial oligoclase grains may have crystallized in larger pools, as the composition of minerals crystallized from melt is also related to the available pore space (Holness and Sawyer, 2008).…”