Advective heat transfer and fabric development in a shallow crustal intrusive granite — the case of Proterozoic Vellaturu granite, south India

Abstract

Syntectonic plutons emplaced in shallow crust often contain intermediate-to low-temperature deformation microstructures but lack a high-temperature, subsolidus deformation fabric, although the relict magmatic fabric is preserved. The Proterozoic Vellaturu granite emplaced at the eastern margin of the northern Nallamalai fold belt, south India during the late phase of regional deformation has a common occurrence of intermediate-to low-temperature deformation fabric, superimposed over magmatic fabric with an internally complex pattern. But high-T subsolidus deformation microstructure and fabric are absent in this pluton. The main crystal plastic deformation and fluid enhanced reaction softening was concentrated along the margin of the granite body. Resulting granite mylonites show Y-maximum c-axis fabric in completely recrystallized quartz ribbons, dynamic recrystallization of perthites, and myrmekite indicative of fabric development under intermediate temperature (∼ 500-400 • C). The weakly-deformed interior shows myrmekite, feldspar microfracturing and limited bulging recrystallization of quartz. The abundance of prism subgrain boundaries is indicative of continuing deformation through low-temperature (∼ 300 • C). The relative rates of cooling influenced by advective heat transfer and deformation of the pluton seem to control the overall subsolidus fabric development. The rapid advective heat transfer from the interior in the early stages of subsolidus cooling was followed by slow cooling through intermediate temperature window as a well-developed phyllosilicate rich mylonitic skin around the granite body slowed down conductive heat loss. Low-T crystal plastic deformation of quartz was effected at a late stage of cooling and deformation of the shallow crustal granite body emplaced within the greenschist facies Nallamalai rocks.

Unraveling an antique subduction process from metamorphic basement around Medellín city, Central Cordillera of Colombian Andes

“…The estimations indicate two different pressure ranges in a narrow temperature (788e642 C), between 8.7 and 7.9 kbar and from 5.5 to 2.8 kbar. These results are compatible with the partial melting of granitic rocks in a shallow crustal level (Saha and Chakraborti, 2007).…”

SHRIMP U–Pb zircon provenance of the Sullavai Group of Pranhita–Godavari Basin and Bairenkonda Quartzite of Cuddapah Basin, with implications for the Southern Indian Proterozoic tectonic architecture

“…The Vinukonda Granite is dated as 1589 Ma The Vellaturu Granite, which has been demonstrated to cut the Nallamali Group (Saha and Chakraborty, 2007), has a reported Rb-Sr model age of ~1575 Ma (Crawford and Compston, 1973). We postulate that this Rb-Sr model age is suspect and that the Vellaturu Granite is younger than 1500 Ma.…”

Intracontinental thrusts and inclined transpression along eastern margin of the East Dharwar craton, India

“…Upper greenschist to amphibolite facies temperature (>450º C) of deformation along the Vellikonda thrust front is supported by the fabric pattern and its interpretation in terms of activation of slip systems in quartz. Deformation induced microstructure and fabric in the syntectonic Vellaturu granite also support deformation at deeper crustal levels in the northeastern NFB, where the amphibolite facies NSB rocks are thrust over the NFB (Saha and Chakraborti 2007). This is in contrast to the low temperature (250-300º C) deformation induced fabric in the Neoproterozoic Kurnool Group either in the footwall of the Palnad nappe in the northeast or close to the Rudravaram line (Meijerink et al 1984) in the west, which is discussed below.…”

“…The Nallamalai rocks in the vicinity of the Vellikonda thrust front have suffered multiple deformation (Saha, 2002;Saha and Chakraborti 2007). Quartz mylonites derived from the Nallamalai quartzites are ubiquitous along the thrust and inclined to reclined folds are common (Fig.3a).…”