The dike swarm of the Karakoram shear zone, Ladakh, NW India: Linking granite source to batholith

Abstract

Debate regarding migration of granitic magmas arises from the fact that the physical link between migmatites and plutons and batholiths is rarely documented. Within the Karakoram shear zone, Ladakh, NW India, the synkinematic transfer of magma can be traced from the anatectic source region to the Karakoram Batholith through a complex dike swarm. Melting is due to water fl uxing at upper-amphibolite facies, producing Miocene hornblende and garnet-two mica leucogranites. The anatectic zone is characterized by synmagmatic folding and shearing of migma tites, and by pervasive, irregular magma migration paths forming injection complexes. These irregular networks are linked to dike networks that are interpreted to represent a magma transfer zone characterized by a range of styles, such as (1) dike swarms parallel to the shear plane, (2) dikes following the two major foliation planes in the shear fabric, (3) magma sheets in conjugate pairs of ductile fractures, or (4) chaotic dike complexes. Network geometries are controlled by the regional stress fi eld, strain distribution, preexisting anisotropies, and rheological contrasts. Dikes give rise to anastomosing systems with rare crosscutting relationships, and with intersections oriented parallel to the dominant mineral stretching lineation. Dikes feed a number of sills, stocks, plutons, and, ultimately, the leucogranitic Karakoram Batholith. We propose that magma developed an interlinked, continuous dike swarm from source to sink by interacting with deformation and preexisting anisotropies. Unlike theoretical and laboratory predictions, dikes are typically oriented at high angles to the maximum shortening axis, and unlike expectations, water-fl uxed melting of the crust produced mobile magma that migrated to form large granitic bodies.

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

“…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].…”

“…As these steep axial planes trending N120 to N140 are sub‐parallel to the mylonitic foliation in the two strands framing the Pangong range it was concluded that anatexis, folding and right‐lateral shearing were coeval [ Weinberg et al , 2009; Weinberg and Mark , 2008]. 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.…”

“…The youngest age group corresponds to a Miocene magmatic episode for which field relations, detailed structural analysis and isotopic analysis suggest widespread partial melting of two major rock sequences, meta‐sedimentary and calc‐alkaline rocks, in the presence of a water‐rich fluid contemporaneously with right‐lateral deformation [ Hasalová et al , 2011; Reichardt and Weinberg , 2012; Reichardt et al , 2010; Weinberg et al , 2009; Weinberg and Mark , 2008]. For example, the North Muglib granite (called « Tangtse granite » by several authors), that crystallized between 18.0 ± 0.4 Ma (U/Pb zircon and titanite, sample TNG148a) [ Reichardt et al , 2010] and 15.1 ± 0.6 Ma (U/Pb zircon, monazite, xenotime, sample P46) [ Phillips and Searle , 2007], is interpreted to have crystallized during right‐lateral shear in a pressure shadow of the competent ∼71 Ma old South Muglib granite [ Reichardt et al , 2010; Weinberg et al , 2009].…”

“…An intimate link has long been proposed between large strike‐slip faults affecting the continental crust and magmatism [e g., Hutton and Reavy , 1992; Leloup et al , 1999, and references therein]. In the KSZ such a link has been proposed based on field and microstructural observations attesting for synkinematic magmatism in the Tangtse area [ Lacassin et al , 2004; Leloup et al , 2011; Reichardt and Weinberg , 2012; Reichardt et al , 2010; Rolland et al , 2009; Valli et al , 2008; Weinberg et al , 2009; Weinberg and Mark , 2008]. The same relationship has been proposed farther southeast in the North Ayilari range, where a mildly deformed dyke that crosscuts the right‐lateral foliation, which has thus been interpreted as synkinematic, yielded a zircon U/Pb age of 22.7 ± 0.1 Ma (Sample C32) [ Lacassin et al , 2004; Valli et al , 2008].…”