Structures of the porphyritic granite and associated metamorphic rocks of east manbhum, bihar, india
01 May 1956-Geological Society of America Bulletin (Geological Society of America)-Vol. 67, Iss: 5, pp 647-670
TL;DR: Porphyritic granite is associated with metamorphic rocks and migmatites in East Manbhum (23°27′45″-23°35′ N. Lat., 86°30′-86°49′ E. Long), India, crops out as a lenticular body showing well-developed planar banding of alternating feldspar phenocrysts and a finer-grained assemblage of quartz, feld spar, and accessories as discussed by the authors.
Abstract: Porphyritic granite, associated with metamorphic rocks and migmatites in East Manbhum (23°27′45″–23°35′ N. Lat.; 86°30′–86°49′ E. Long.), India, crops out as a lenticular body showing well-developed planar banding of alternating feldspar phenocrysts and a finer-grained assemblage of quartz, feldspar, and accessories. Arrangement of phenocrysts within the banding may be random, or they may show perceptible lineation. Lineation, foliation, and joints are primary. Regional distribution of flow layers suggests an inclined lens, with two secondary domes, one at Raghunathpur and the other at Bero. Lineation, generally within a few degrees from the horizontal, parallels b , except in the two domes where it parallels a. The porphyritic granite is concordant for the greater length except toward the eastern margin of the exposure. North-northeast of Raghunathpur, it is discordant with the structural trends of the country rocks. There are a few small-scale discordances where the granite boundary takes sharp turns, leaving the schistose alignments of the country rocks abutting against the boundary. Spatial distribution of foliation, lineation, and joints of the porphyritic granite is independent of that of the metamorphic and migmatitic country rocks. Wall rocks have been mylonitized, and three joint systems developed. Distribution and concentration of joints prove that the effect of tension was comparatively greater close to the contact. Compression diminishes more slowly and is effective over a greater distance. Lineation in the country rocks may be parallel to both a and b. It is chiefly of two types produced by (1) microfolds of different dimensions, or by (2) alignment of elongated minerals such as quartz or sillimanite. Biotite fabric of the metamorphic rocks is not indicative of their tectonic trend as the mineral is a result of neocrystallization. Quartz-axes diagrams show peripheral ac girdles with maxima I, II, and V, and occasionally an ac ⁁ bc diagonal girdle. The orientation in the metamorphic rocks, especially as they belong to a high grade of regional metamorphism, presumably originated in solid flow by translation, chiefly along megascopic s planes. Fabric of migmatites, ultramigmatitic granite gneiss, and two types of quartz in leptynites (granulites) are very similar to that of the metasediments, and their orientation is ascribed to mimetic retention of tectonitic orientation during metasomatic recrystallization. Country rocks show simultaneous fabric reconstitution near the porphyritic granite. Along the granite contact the fabric is symmetrical about the bisectrices of the shear joints caused by intrusion of porphyritic granite. The intrusion also produced minor secondary folds or intensification of folding. Quartz fabric of the porphyritic granite is independent, with regard to its pattern and relation to geographic coordinates, of the country rock fabric.
TL;DR: In this paper, a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent, diapiric emplacement and expansion, or regional deformation post-dating emplacements.
Abstract: Foliations in granitoids can form by magmatic flow, ‘submagmatic flow’, high-temperature solid-state deformation and moderate- to low-temperature solid-state deformation. A review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent, diapiric emplacement and expansion, emplacement during regional deformation, or regional deformation post-dating emplacement. However, a magmatic origin is favoured for foliations defined by the alignment of igneous, commonly euhedral minerals, particularly where the foliation is parallel to internal or external pluton contacts. Foliations formed during expansion or ‘ballooning’ of diapirs may be strictly magmatic in origin, although some studies suggest that solid-state deformation also may occur. If so, we would hope to find evidence of deformation of crystal-melt systems, and that the solid-state deformation occurred at high temperatures. The inference of syntectonic foliations is most convincing where magmatic and high-temperature solid-state foliations are subparallel, these foliations are continuous with regionally developed foliations in the wall rocks, synkinematic porphyroblasts are present in the wallrocks, and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage. A strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals, no alignment of igneous minerals occurs, the foliation is locally at high angles to pluton-wallrock contacts, and the foliation is continuous with a regionally developed cleavage.
TL;DR: K-feldspar megacrysts in granitoid plutons have been interpreted as either phenocrysts or porphyroblasts as mentioned in this paper, which can be explained by growth or mixing in magma before a globule of that magma or a fragment of the resulting igneous rock was incorporated as an enclave.
Abstract: K-feldspar megacrysts in granitoid plutons have been interpreted as either phenocrysts or porphyroblasts. Most of the microstructural, mineralogical and chemical evidence (e.g., shape, alignment, concentration, Ba content, zoning, inclusions, and twinning) favours a phenocryst origin. The main features that have been used to support a porphyroblast origin are occurrence of megacrysts: (1) across aplite vein boundaries, (2) in country rocks, and (3) in or across boundaries of microgranitoid enclaves (mafic inclusions). However, these features can be explained by the phenocryst hypothesis. In particular, megacrysts in microgranitoid enclaves can be explained by growth or mixing in magma before a globule of that magma or a fragment of the resulting igneous rock was incorporated as an enclave. All available evidence favours or is consistent with a phenocryst origin for K-feldspar megacrysts in granitoid rocks and their enclaves. The large size of the megacrysts is evidently due to nucleation difficulties for K-feldspar in granitic melts. Though K-feldspar is commonly the last mineral to begin crystallizing in granitic magmas, abundant melt is still present at that stage, allowing sufficient space for the megacrysts to grow. The reason for the common lack of megacrysts in volcanic rocks may be that the phenocrysts do not grow large enough to be called “megacrysts” until the magma contains such a high proportion of crystals that it cannot erupt.
TL;DR: The Mesoproterozoic Sausar Mobile Belt (SMB), the Chhotanagpur Granite Gneiss Complex (CGGC), and the gneissic complex of Northeast India represent the wider southern belt.
Abstract: The ESE-WNW trending Central Indian Tectonic Zone (CITZ) divides the Indian subcontinent into southern and northern crustal provinces. Its narrow northern belt consists of closed early Proterozoic Mahakoshal rift zone, which is presently confined between two Moho reaching faults that characterize the Son-Narmada (SONA) lineament. The Mesoproterozoic Sausar Mobile Belt (SMB), the Chhotanagpur Granite Gneiss Complex (CGGC) and the Gneissic Complex of Northeast India represent the wider southern belt. The Mahakoshal rift belt has a thick pile of mafic-ultramafic rocks and possibly oceanic components. The basin closed due to a south-directed subduction creating a magmatic arc that was sutured to its southern boundary during 1.8-1.7 Ga. A south dipping ductile shear zone, known as the Son-Narmada South Fault affects both its supracrustals and intrusive granitoids. The Son-Narmada South Fault generally delimits the northern boundary of the Gondwana Basins, whereas the Son-Narmada North Fault, delineates the northern boundary of the Mahakoshal Mobile Belt and the southern boundary of the Meso-Neoproterozoic Vindhyan Basin. The SMB, representing the southern belt of the CITZ in the central sector, comprises strongly folded and metamorphosed non-volcanic and Mn-rich Sausar sediments, intermixed with reworked basement gneisses, migmatites and granulites representing the pre-Sausar assemblage. The Sausar Orogeny closed during terminal Mesoproterozoic (ca. 1.0 Ga). On the other hand, metamorphism in the basement gneiss, migmatites, and northern and southern granulite belts pre-dates the Sausar Orogeny. The earliest structure of the SMB truncates and rotates the N-S oriented structures of the southward-located Sakoli and the Dongargarh belts of Neoarchaean-Palaeoproterozoic age. The northern granulite belt of the SMB and the CGGC, and the high-grade supracrustals from the Sonapahar area in NE India, all indicate very high P-T conditions during 1.7-1.5 Ga peak metamorphism, which was followed by decompressional cooling. The setting resembles continental collision, which was followed by tectonic exhumation and denudation. On the other hand, the southern granulite belt from the SMB documents a late Archaean (2.6 Ga) event and a later 1.4 Ga event. The Dalma Volcanic Belt located close to the southern tectonized boundary of the CGGC belt, and possibly the assemblage of copious mafic enclaves within the ca. 550 Ma granite plutons from NE India, represent back arc-like setting during 1.6-1.5 Ga. The setting is caused due to the southward subduction of the north Indian block beneath the southern block leading to continental collision. The metasedimentary belt, located further to the south of the Dalma Volcanic Belt and flanking the Archaean Singbhum Craton to the north, on the other hand, is Palaeoproterozoic in age, but has been metamorphosed during ca. 1.6 Ga because of the northern collision-related orogeny. Reworking and incorporation of basement gneisses and granulites from the mid-crust level to the younger Sausar cover sediments denote renewed spell of thrust tectonics that remobilized the collision zone rocks of the earlier orogeny. The tectonic transport was from the north to the south with north-dipping thrusts. Thus there was a reversal in the direction of convergence from the preceding orogeny. The Sausar Orogeny closed during ca. 1.0 Ga. It caused anatectic event producing granitoid plutons, migmatites and charnockites in the CGGC belt, and migmatites in the Gneissic Complex in NE India.
TL;DR: A great deal of evidence, involving many different factors, favours a magmatic/phenocrystic origin for K-feldspar megacrysts in granites, namely simple twinning, oscillatory zoning, euhedral plagioclase inclusions, and concentric, crystallographically controlled arrangements of inclusions.
Abstract: Various petrologists have suggested that K-feldspar megacrysts grow in granites that are extensively crystallized, even at subsolidus conditions. However, experimental evidence indicates that, though K-feldspar nucleates relatively late in the crystallization history, abundant liquid is available for development of large crystals. A great deal of evidence, involving many different factors, favours a magmatic/phenocrystic origin for K-feldspar megacrysts in granites, namely simple twinning, oscillatory zoning, euhedral plagioclase inclusions, and concentric, crystallographically controlled arrangements of inclusions. In addition, abundant evidence has been presented of (1) mechanical accumulation of K-feldspar megacrysts in granites, (2) alignment of megacrysts and megacryst concentrations in magmatic flow foliations, (3) involvement of megacrysts in zones of magma mixing in granite plutons, and (4) occurrence of megacrysts in some volcanic rocks, implying that the megacrysts were suspended in enough liquid to be moved without fracturing or plastic deformation. Detailed trace element and isotopic data also indicate that megacrysts can move between coexisting felsic and more mafic magmas. Irregular overgrowths on megacrysts are consistent with continued magmatic growth after euhedral megacrystic growth ceased, the overgrowths being impeded by simultaneously crystallizing quartz and feldspar grains.
TL;DR: In this paper, the Chotanagpur Granite Gneiss Complex (CGGC) has been reviewed with a view to identifying the different metamorphic episodes; developing an event stratigraphy in the high-grade blocks; and correlating the different meta-events with the globally extensive orogenic processes.
Abstract: Abstract Geological information on the Chotanagpur Granite Gneiss Complex (CGGC) has been reviewed with a view to: (a) identifying the different metamorphic episodes; (b) developing an event stratigraphy in the high-grade blocks; and (c) correlating the different metamorphic episodes with the globally extensive orogenic processes. Integrating the existing geological information, the geological evolution of the high-grade block of the CGGC has been divided into four stages associated with four distinct metamorphic events (MI−MIV). The earliest metamorphic event (MI) that is recorded in granulite enclaves in the regionally extensive felsic gneisses culminated in ultrahigh temperatures (>900 °C at c. 5–8 kbar) at around 1.87 Ga. In the second stage, voluminous felsic magmas were intruded – the MI granulites – and were metamorphosed to form migmatitic felsic gneisses (MII) within about 1.66–1.55 Ga. The third stage witnessed intrusions of a suite of anorthosite and porphyritic granitoids (c. 1.55–1.51 Ga), followed by high-grade metamorphism (700±50 °C, 6.5±1 kbar, MIII) during approximately 1.2–0.93 Ga. The fourth stage (MIV) is marked by the intrusion of a suite of mafic dykes, followed by infiltration-driven metamorphism (600–750 °C at 7±1) during 0.87–0.78 Ga. The proposed metamorphic events have been correlated with the supercontinental cycles in the Proterozoic time.
Cites background from "Structures of the porphyritic grani..."
...2), has been studied by a number of workers (Dunn 1929; Sen 1956, 1959; Baidya et al. 1987, 1989; Ray Barman et al. 1994)....