Metasedimentary Enclaves Research Articles

Geochemistry and Origin of Megacrystic Granitoid Rocks from Eastern Ghats Granulite Belt

The Eastern Ghats Granulite Belt (EGGB) forms part of a continuous Precambrian metamorphic terrain in Gondwana. It is characterised by widespread development of an Archaean khondalite suite of metasedimentary rocks, Archaean to Late-Proterozoic charnockites and Late Proterozoic anorthositic, granitic and syenitic emplacements. A 1900 Ma megacrystic granitoid suite, containing varying proportions of charnockites and granites, forms an important and widely distributed litho-unit in the central khondalite and eastern migmatite zones of the EGGB. It contains metasedimentary enclaves, megacrystic K-feldspar, quartz, plagioclase ovoids, biotite, garnet (porphyroblasts and coronas), apatite, zircon, ilmenite, magnetite, etc. Hypersthene is present in the charnockite phase. Monazite is present in some garnet-free granites. It is characterised by low Na 2O/K 2O ratios, high alumina saturation index, CaO, MgO, and ÝREE, negative correlation of TiO 2, Al 2O 3, Fe 2O 3 t, MgO, MnO, CaO, P 2O 5, Ba, Sr, Zr and V with SiO 2, positive correlation of K 2O, REE, Th and Rb with SiO 2, fractionated LREE, relatively flat HREE and negative Eu anomalies. The data suggest S-type nature of the suite. Fractionation of the granitic magma and local variations in pH 2O and fCO 2 caused the formation of megacrystic charnockites. Formation of the corona garnet is related to the reworking of the suite during late Proterozoic (ca. 1250 Ma) isothermal decompression associated with channelised CO 2-rich fluid flux along narrow shear zones.

Structural analysis of metasedimentary enclaves: Implications for tectonic evolution and granite petrogenesis in the southern Lachlan Fold Belt, Australia: Comment and Reply

Structural analysis of metasedimentary enclaves: Implications for tectonic evolution and granite petrogenesis in the southern Lachlan Fold Belt, Australia: Comment and Reply

Structural analysis of metasedimentary enclaves: Implications for tectonic evolution and granite petrogenesis in the southern Lachlan Fold Belt, Australia

Schistose, upper amphibolite facies, metasedimentary enclaves from granites across the southern Lachlan Fold Belt of southeastern Australia have a common structural history. The middle crustal region, from which the enclaves and their host granites derived, is assumed therefore to be structurally uniform across the fold belt. The uniform nature of the crustal component represented by the metasedimentary enclaves casts doubts on the existence of suspect or Proterozoic microcontinental blocks within the terrane collage. A proposed crustal cross section for the southern Lachlan Fold Belt shows a tectonically thickened Paleozoic section overlying an oceanic substrate. An Ordovician sedimentary component was involved in the generation of both S- and I-type granites.

Igneous and Metamorphic Enclaves in the S-type Deddick Granodiorite, Lachlan Fold Belt, SE Australia: Petrographic, Geochemical and Nd-Sr Isotopic Evidence for Crustal Melting and Magma Mixing

The Deddick Granodiorite, a mafic S-type pluton in the Lachlan INTRODUCTION Fold Belt, contains abundant enclaves. Most are derived from Enclaves in granites and volcanic rocks can provide high-grade metasediments of pelitic–psammitic composition, and petrogenetic information not readily available from their migmatites are common. Among these, discrete fragments of host rocks (Didier, 1973; Didier & Barbarin, 1991). For melanosomes rich in cordierite and garnet are restites from partial example, metasedimentary enclaves can place constraints melting. However, compositional and Nd–Sr isotopic data indicate on the age, composition and deformational history of they are not restite in equilibrium with the melt component of the magma source regions within continental crust (Fleming, host magma, but may have formed part of a spectrum of diverse 1991; Steele et al., 1991). Enclaves formed by acmagma source lithologies. Alternatively, they may be accidental cumulation of early phenocrysts (‘autoliths’) document xenoliths. Regardless of their origin, data for these enclaves suggest the early evolution of the host magma, whereas others their precursors could represent deeply buried age equivalents of the may represent commingled magmas and illustrate the ubiquitous Ordovician turbidites of the Lachlan Fold Belt. Three possible role of hybridization (Didier, 1987). types of microgranular enclaves with igneous textures are disEnclave suites in the relatively mafic S-type granitoids tinguished. The most common type are small and rounded mafic of SE Australia are often diverse, including abundant enclaves of tonalitic composition. They carry xenocrysts derived from enclaves of metasedimentary origin and microgranular the host magma, and some contain high-Mg pyroxene derived from enclaves (Phillips et al., 1981; Price, 1983; Chen et al., a mafic magma. Isotopic data form an array with eNd from –6 to 1989; Wyborn et al., 1991). The metasedimentary en–12, and Sr/Sr from 0·7130 to 0·7167 (host rock –10 and claves are lithologically diverse, including gneissic– 0·715, respectively). These enclaves formed as globules of hybrid migmatitic types and biotite-rich ‘surmicaceous’ types. mafic magma commingled with, and contaminated by, the more Apart from their potential as indicators of crustal structure felsic host magma. Other enclave types were derived from disrupted and composition, they are of interest as possible fragments syn-plutonic dykes having a distinct isotopic composition, and from of source material and/or refractory residue from which a cogenetic marginal facies of the host. peraluminous melts were generated and extracted. By

Inherited deformation structures in metasedimentary enclaves in granites as “windows” into deeper levels of the crust

Deformation structures preserved in metasedimentary enclaves in granites can provide information on the deformation history of the granite source region up to the time of melt generation, particularly where the inclusions record more deformations than were present in the country rock at the time of emplacement. The data provide a “window” to the source that is additional to that derived from petrology and geochemistry. The improved view can give extra information about the nature of the source rocks and their structural/tectonic history. If applied across a wide region such as an orogenic belt, the approach could provide fundamental information on large expanses of unexposed crust, and could help delineate suspected (or unsuspected) boundaries between terranes or basement terranes. Examples from two granitic plutons in the Palaeozoic Lachlan Foldbelt of southeastern Australia support independent assertions that at least in places the foldbelt structurally overlies pre-Ordovician (Late Proterozoic? Cambrian?) continental crust rather than oceanic crust. Alternatively, it overlies Ordovician continental crust, the structural and chemical composition of which are different from Ordovician rocks at surface.

Spinel + Quartz Granites and Associated Metasedimentary Enclaves from the Taltson Magmatic Zone, Alberta, Canada: A View into the Root Zone of a High-Temperature, S-Type Granitic Batholith

The Taltson magmatic zone (TMZ) makes up the southern part of the early Proterozoic (ca. 1.9-2.0 Ga) Ta l t son-The lon orogen, a nor th-south trending belt that extends for more than 2000 km along strike from central Alberta to the Arctic Ocean (Hoffman, 1988; Ross et al., 1991). The TMZ is dominated by felsic plutonic rooks, in particular by peraluminous grantoids (Bostook et al., 1987; Goff et al., 1986). The present study focuses on the pe t ro logy of a spectacular outcropping of one such granitoid body and its associated enclaves. The rooks are unusual in that both the granites and the enclaves contain the high-temperature mineral assemblage, spinelquartz-cordierite-garnet+sillimanite. The aim of this study is to document the pressure-temperature conditions indicated by this assemblage and to assess the magmatic and metamorphic processes leading to its development. Descriptions of rooks from other parts of the TMZ (Bostock et al., 1987; Grover et al., 1993) suggest that processes documented at this outcrop may be representative of those that operated over much of the TMZ.

Tectonic Setting, Petrochemistry and Tungsten Metallogeny of the Sewariya Granite in the South Delhi Fold Belt, Rajasthan

Abstract The Scwariya granite, 'largest of the Erinpura granite intrusions' occurs along the western margin of the Middle-Late Proterozoic Soulh Delhi Fold Bell. Synkincmalic with the Delhi orogeny, the granite body intrudes a rifled back arc type basin (Barotiya Formation) and hosts W ± Sn ± Li mincralization in pneumatolytic quartz veins. Innumcrable metasedimentary enclaves occur which are traversed by syn-to post-tectonic ductile shear zones. Geochemistry of Ihe granite suggests it to be a well differntiated, calc-alkliline, S-typc metallogcnetically specialised orogenic massif. The nature of the granite and the host supracrustals and Ihe tectonic discriminant diagrams indicate that it may have evoled in an arc-related environment. W ± Sn ± Li mineralization in this granite, Pb-Zn-Cu and Zn-Cu mineralization in the contiguous units of Barotiya and Sendra formations respectively suggests a conspicuous metallogenic zoning. The Sewariya granite forms a central link to the major Sn-W hosted granite rocks in Rajasthan Haryana sector that lie along a 500 km long NNE-SSW trending lineament. Although associated with felsic plutonic suites of contrasting geotectonic environments, W -Sn mineralizalion along this belt defines a single metallogenic province, a large part of which is unexplored.

The origin of granite: The role and source of water in the evolution of granitic magmas

Temperature and water content are the two most important parameters in the formation of granitic magmas. Evidence from volcanic and plutonic lithologies suggests that water contents of 2 to 4 wt. % are present in most silicic magmas. Calculations based on the stability of biotite yield water fugacities within the melt phase from about 500 to 2,000 bars, although these calculations determine the log f H 2 O and the effective uncertainty of ±0.5 log units yields a large absolute uncertainty. Comparison with crystallization experiments demonstrates that less than 2% water would require significant percentages of crystallization at temperatures above 900 °C with liquidus temperatures about 1000 °C. Water contents greatly in excess of 4 wt. % would mean that the magma would become vapor saturated at high pressures and would tend to crystallize during ascent to a fine-grained granite before reaching shallow depths. The main sources of water for magma generation are the dehydration of hydrous silicates within the crust and volatiles transported into the crust from subducted oceanic crust and upper mantle in the form of hydrous basalts and andesites. Dehydration reactions of muscovite, biotite, and horn-blende are of particular significance. Anatectic granites may be partially classified in terms of the probable dehydration reaction responsible for their generation. Melts generated from muscovite dehydration are relatively cool, peraluminous in composition, high in K/Na ratio, and generally high in initial 87 Sr/ 86 Sr and delta 18 O. Biotite-generated melts tend to be higher in temperatures, peraluminous to meta-aluminous in composition, moderately high in K/Na ratio, and relatively high in strontium and oxygen ratios. Both types may contain metasedimentary enclaves. Granitoids generated by hornblende dehydration would be much higher in initial temperature, peralka-line to meta-aluminous in composition, and lower in K/Na ratio and in general would have lower strontium and oxygen ratios. Volatiles deposited in hydrothermally altered oceanic crust and upper mantle will be released during subduction in the form of hydrous basalts and andesites. These melts are important energy-transfer mechanisms to drive anatexis within the crust. If these melts encounter a silicic magma chamber when intruding the crust, they will be trapped below the lower-density granitic melt. While partially quenching against the cooler melt, they may release volatiles and heat, which will aid in the melting process. Granites generated in this way will vary in their geochemical properties, depending on the relative importance of crustal versus mantle magma systems. In general, they will have moderate to high initial temperatures, be meta-aluminous in composition, be variable but somewhat low in K/Na ratio, and have lower initial strontium and delta 18 O ratios than will melts derived completely from the crust. The fact that most terrestrial spreading centers are subaqueous thus may aid in the formation of a thick granitic continental crust.

The Southern High-Grade Margin of the Dharwar Craton

The amphibolite-facies to granulite-facies transition at the southern margin of the Dharwar craton has been studied in the Krishnagiri, Satnur-Halaguru, and Kushalnagar areas of southern India. In all three areas the transition appears to be a progressive metamorphic overprint on cratonal gneisses and their mafic and metasedimentary enclaves, without major structural interruption. In the Kabbal-Satnur-BR Hills section of Karnataka, a high-grade charnockite massif with pronounced Rb depletion is the culmination of an apparently continuous increase of metamorphic grade southward. In this and the Kushalnagar areas, increase of paleopressure from near 6 to near 8 kbar with increasing grade indicates a depth-zone relationship of the amphibolite and granulite facies. Incipient charnockite replacing Peninsular Gneiss first appears along N-S shears parallel to the regional grain of the craton. Low-P(), high vapors were instrumental in the creation of orthopyroxene. Introduction from a deep source, either a decarbonating mantle, basaltic underplate, or deeply buried sediments, was facilitated by the N-S deformation system. A deformed continental margin or infracontinental basin in the latest Archean is a plausible setting for the metamorphism. Great crustal thickening, perhaps with entrainment of shelf or basin sediments, was effected by overthrusting, the record of which may be preserved in an early isoclinal foliation and in fold-interference patterns in the southern cratonal margin. Subsequent transcurrent shearing facilitated outgas-sing of the deep crust and upper mantle, and rising vapors transported heat, , and K to middle levels of a thickened crust, which resulted in anatectic melting. Eventual uplift and erosion exposed a 6-8 kbar paleopressure surface at the southern cratonal fringe. The weakened and segmented continental shelf or platform south of the present craton was prone to later remobilization and retrogression, in the manner of a typical "mobile belt." This view of the relationship of the Dharwar craton to the southern high-grade terrain provides no support for the concept that granulite facies terrains of the general aspect of the South Indian massifs everywhere underlie the interior of the craton. The 2-3 kbar terrain of the cratonal interior was not greatly thickened by overthrusting nor magmatic underplating during the 2.6 Ga high-grade event that affected the southern cratonal margin. It is, however, possible that granulite-grade roots of an older metamorphic cycle underlie the cratonal interior.

Mineral parageneses and metamorphic reactions in metasedimentary enclaves from the Archaean Gneiss Complex of north-west India

AbstractThree metasedimentary enclaves up to a kilometre in length of contrasting compositions within the polymetamorphic Banded Gneissic Complex (> 2580Ma) have been studied for their mineral parageneses and metamorphic conditions. The largest enclave, consisting of kyanite-chloritoid-muscovite schist with quartz or corundum, and kyanite-fuchsite-corundum ± diaspore, was metamorphosed at most under lower amphibolite conditions, and is thus not isofacial with the surrounding schists and gneisses (of the ‘basement’ complex) which reached sillimanite-grade metamorphism in the last orogenic cycle (Aravalli: 1650–950Ma Orogeny) in Rajasthan.The second enclave is a calc-silicate rock which occurs as a small lens. The presence of two generations of wollastonite which formed during different metamorphic events in the calcite-quartz-grossularite-anorthite-clinopyroxene assemblage indicates polymetamorphism.The third enclave is a metabasic rock which records a complete polymetamorphic history in discontinuous zones in garnet coexisting with hornblende-chlorite-plagioclase-quartz±epidote. To explain the garnet zoning a model involving partial resorption of early garnet during the initial recrystallization stage of superimposed regional metamorphism is preferred to the alternative based on a single prograde metamorphism and retrogression.The mineralogy of the calc-silicate and metabasic enclaves gives a recrystallization temperature of c. 700°C and a pressure in the range of 8–3 kbar during the second metamorphism.

The problem of the Precambrian basement in Rajasthan, western India

The Banded Gneissic Complex (BGC) of Rajasthan, considered to form the basement underlying the Precambrian (Proterozoic) Aravalli metasediments, shows an erosion surface marked by a conglomerate and an angular unconformity, with gneissic foliation crossing the metasedimentary bands at only a few places. The BGC is a composite gneiss, evolved by extensive migmatization of metasedimentary rocks of diverse composition, and possibly metaigneous rocks. The contact between the BGC and the Aravalli rocks is a gently curved surface, whereas the gneissic foliation, as well as the large-scale metasedimentary enclaves within the gneissic complex, show all the intricate patterns of super-imposed folding traceable in the Aravalli rocks. This implies that the “basement” gneisses have been involved in ductile deformation with the Aravalli rocks, the migmatization being synkinematic with the first deformation in the latter. All these apparently conflicting lines of evidence can be resolved if the gneisses, as we see them now, represent not the original, but the mobilized basement, with the BGC-Aravalli boundary representing, for a large part, a migmatite front, rather than the original basement-cover interface. Only at a few places was there a chance of the original basement escaping mobilization and thus, little chance of identifying this original interface.

Metamorphism of cordierite gneisses from the Bangalore region of the Indian Archean

Mineral compositions from cordierite-bearing metasedimentary enclaves adjacent to the Closepet granite in the Bangalore region of the South Indian Craton indicate peak metamorphism in the range 4.5–5.0 kb, 690–730°C. P H 2O is found to decrease towards the south and those assemblages which lie south of the charnockite line are formed under conditions of P H 2O < 0.3 P total. 250 km north Bangalore, andalusite-bearing assemblages from the northern margin of the Closepet granite indicate peak metamorphic conditions in the range 2.6–3.8 kb, 520–620°C. The inferred piezotacrmic array is strongly convex towards the temperature axis and may represent a transient geotherm in the upper crust related to the emplacement of the Closepet granite or to volatile activity along the margins of the granite.

Bathymetry and faults of Gulf of California : Rusnak Gene A., Robert L. Fisher and Francis P. Shepard, 1964. In: Marine geology of the Gulf of California, a symposium, Amer. Ass. Peetrol. Geol., Memoir. (Edited by van Andel T. and George G. Shor Jr.), 3: 59–75

The Maluwe basin, north-adjacent to the Sunyani basin, is the northernmost of the northeast-trending Eburnean volcaniclastic depositories in Ghana. These basins are separated from one another by remnants of Eoeburnean crust, all formed during the evolution of an arc-backarc basins complex in a Palaeoproterozoic intraoceanic environment. The Bolé–Nangodi belt terrane to the northwest, of mostly Eoeburnean crust is fault bounded with the Maluwe basin along the northeast-trending Bolé–Navrongo fault zone. The stratigraphic sequence, which was the key to unravelling the structural evolution of the study area, was established by means of field observations aided by precision SHRIMP geochronology. The quartzitic, pelitic, quartzofeldspathic and granitic gneisses of the Eoeburnean crust (>2150 Ma) experienced complex metamorphic mineral growth and migmatitization, mostly under static crustal conditions and were subjected to several deformation episodes. The foliated mafic and metasedimentary enclaves within the Ifanteyire granite establish deformation to have taken place prior to ∼2195 Ma, while the tectonically emplaced Kuri amphibolites within the 2187-Ma gneissic Gondo granite indicate a stage of rifting followed by collision. Deformation of granite dykes in the Gondo granites at ∼2150 Ma concluded the development of the Eoeburnean orogenic cycle (DEE).The Sawla Suite, contemporaneous with the deposition of the Maluwe Group, intruded the tectonic exhumed Bolé–Nangodi terrane during extension between ∼2137 and 2125 Ma. The rifting separated the Abulembire fragment from the Bolé–Nangodi terrane. During subsequent northwestward subduction of young back-arc basin oceanic crust the volcaniclastic strata of the Maluwe Group and Sawla granitoids were deformed (DE1) under chlorite/sericite greenschist-grade conditions. The NE-trending folds had subhorizontal axes and subvertical axial planes. Simultaneous to the DE1 orogenesis the molasses of the Banda Group was deposited concordantly on the submerged Sunyani strata after a hiatus of 20 million years. After cessation of the NW–SE-directed compression the early Tanina Suite intruded as batholiths, dykes and sheets and produced garnet, staurolite, sillimanite and kyanite in their thermal aureoles. Docking of the Sunyani basin produced the DE2 thrust related folding and stacking in the deformed and granitoid invaded Maluwe basin as well as the single stage sin- and anticlinoria in the Sunyani and Banda Groups. In the Maluwe basin the Abulembire fragment acted as a resistor and the approaching front rotated anticlockwise and clockwise around the barrier to form west- and north-directed piggy-back thrust-stacking and deformation of the Tanina Suite granitoids. Due to the low metamorphic conditions the DE2 fabric is limited to crenulation cleavages in the more psammitic and pelitic units. The fold axes are double plunging (N–S and E–W) up to 60° with the axial planar fabric subvertical. Post-D2 tectonic relaxation has allowed the emplacement of the last Tanina Suite calc-alkaline melts and was succeeded by N–S extension fracturing (DE3) along which mantle derived Wakawaka gabbroids and syenite intruded. The DE1 folding occurred between 2125 and 2122 Ma and DE2 before 2119 Ma. The tectonic relaxation occurred at 2118 Ma. Around 2100 Ma, NE–SW directed strike-slip shearing (DE4), fractured the Bolé–Nangodi terrane and enhanced the basin-belt boundary. Along the boundary, the displacement was dextral along vertical faults but, southward, it became more east-over-west thrust related. Associated tension gashes are filled with vein quartz and pegmatite and typical of the brittle sector of the crust. Tectonism in this part of the intraoceanic accretionary arc back-arc complex was concluded by limited, right-lateral strike-slip (DE5) movement which formed some breccias.