Dissertations / Theses on the topic 'Rocks, Metamorphic Schists. Petrology'

This thesis describes an attempt to develop a pioneering method for the analysis of oxygen isotopes in metamorphic rocks. This technique is then applied to a suite of metapelites from Massachusetts, U.S.A. with the aim of investigating metamorphic history. The study of oxygen isotopes is a rapid and efficient way of deciphering the reaction history of a metamorphic rock, and they are particularly useful for quantifying the role of fluids during metamorphism. Technological advances have given the opportunity to develop a new laser fluorination facility capable of in-situ oxygen isotope analysis on the 100μm scale. The use of UV laser ablation coupled with helium carrier flow and isotope ratio mass spectrometry gives the potential for liberation, transfer and analysis of nanomoles of oxygen. This analytical technique is developed herein, and applied to garnets from high alumina metapelites of the Hoosac Schist of Western Massachusetts. These large garnets contain concentric unconformity textures which are attributed to at least two metamorphic events. Core-rim zoning profiles from three Hoosac garnets has been accomplished. Metamorphic modelling in the complex chemical system Na₂O-CaO-MnO-K₂O-FeO-MgO-Al₂O₃- SiO₂-H₂O has yielded P-T estimates for garnet cores of 520°C and 8.5 kbar, and rims at 590°C and 8-10kbar. Within this framework, a new approach enables calculation of oxygen isotope shifts with reaction progress in the presence of a non-equilibrium fluid. Fitted profiles from the Hoosac garnets imply prograde core growth during inflow of external low-δ¹⁸O fluid, and calculations suggest a minimum time integrated fluid flux for the first garnet growth event of the order of 0.2 cm³/cm², some four to five orders of magnitude less than other New England studies.

In the Paleoproterozoic Trans-Hudson Orogen, a well exposed section of mid-crust on Hall Peninsula, southeastern Baffin Island, offers an opportunity to improve our understanding of mid-crustal tectonothermal processes in hot, collisional orogens. Additionally, more robust age constraints on the tectonic history of Hall Peninsula are important for plate tectonic reconstructions of the North Atlantic region. Recent mapping shows that the section comprises Archean crystalline basement overlain by Paleoproterozoic supracrustal rocks, which host felsic plutons on the western peninsula. There is a westward increase in peak metamorphic grade, from amphibolite- to granulite-facies, and three regional deformation events are recognized (D1, 2, 3). Equilibrium phase diagram modeling constrained by garnet compositions in pelite indicates peak conditions of ~720–740°C on the eastern peninsula and ~850°C further west, with pressures of ~6.25–7.35 kbar. Modeling and petrographical evidence suggest subsequent cooling, decompression, growth of retrograde biotite and, on the eastern peninsula, retrograde muscovite. In situ U-Pb monazite dating (~450 analyses) and U-Pb zircon depth profiling (~90 analyses) resolve the timing of regional metamorphism and crustal shortening between ca. 1860–1820 Ma, coincident with the accretion of crustal blocks and arc terranes during the amalgamation of the orogenic upper (Churchill) plate. Regionally-occurring ca. 1800–1750 Ma monazite domains and zircon rims are interpreted to result from fluid-assisted dissolution-reprecipitation. They likely record the terminal collision with the lower-plate Superior craton and post-orogenic thermal activity, possibly related to the emplacement of pegmatitic syenogranite dykes. The new data strengthen formerly tentative correlations with southern Baffin Island, West Greenland and northern Labrador. 40Ar/39Ar thermochronology on muscovite, biotite and phlogopite suggests that Hall Peninsula underwent slow cooling at rates of ~1–2.5ºC/Myr after peak metamorphism, remaining hotter than ~400°C until ca. 1670–1660 Ma. Analogous thermochronological ages from elsewhere in the Trans-Hudson Orogen imply orogen-wide slow cooling. Despite significant crustal thickening and elevated paleotemperatures, the Hall Peninsula crustal section does not record evidence of orogenic collapse, implying that it may not be a hallmark of all hot, thickened orogens.

Rocks of the Quesnel Lake area are divided into three units: unit 1, a continental margin sequence; unit 2, The Crooked Amphibolite (an ocean-floor sequence); and unit 3, the Quesnel sedimentary and volcanic sequence. Two conglomerate localities within unit 3 contain clasts identified as being derived from deformed rocks of units 1 and 2. Deformation of the combined package of units 1 and 2 must have accompanied the emplacement of unit 2 onto unit 1 sometime between the deposition of unit 2 (Mississippian - Permian) and the deposition of unit 3 (Triassic - Jurassic). Rocks of unit 1 have been divided by earlier workers into the Barkerville and Cariboo terranes, separated by the Pleasant Valley Thrust. An extensive review shows that the two terranes are stratigraphically similar and share most of their structural history. The Pleasant Valley Thrust, if it exists, is an extremely early structure. These data do not satisfy the criteria for naming these units 'terranes'. The rocks of unit 1 and 2 experienced an extra phase of deformation not seen in rocks of unit 3. A total of five phases of folding are present. Phases 1 through 4 are approximately coaxial with northwest axes and variably oriented axial planes. Phase 5 has northeast trending axes and vertical axial planes. F₁ is seen in units 1 and 2 only and is visible in outcrop as rootless isoclinal folds and a transposed foliation. In thin section, S₁ is only preserved within the earliest garnet porphyroblasts. F₂ folding is the major deformational event. Peak metamorphism accompanied and outlasted it. Major F₂ folds are present in the field and are accompanied by an axial planar foliation. In thin section, S₂ wraps around earlier porphyroblasts but is overgrown by later ones (staurolite, kyanite). F₃ folding is responsible for the major map-scale structures. It postdated the peak of metamorphism and isograds axe folded by it. In thin section S₃ is commonly a crenulation cleavage or transposed foliation. Some late mineral growth accompanied the early stages of F₃. F₄ and F₅ are buckle folds and kinks and may be conjugate fold sets from a single deformational event. They are not generally visible in thin section. The assemblage silica - garnet - aluminosilicate - mica (SGAM) is common in amphibolite grade meta-pelitic rocks, and can be used as a thermobarometer if the activities of muscovite and biotite can be calculated accurately. A new method of calculating the ideal activity of mica components is proposed. Standard models do not adequately account for the degree of coupled substitution that takes place. The proposed method stores the site occupancies in a 4-dimensional array and manipulates the entries to satisfy three criteria. 1: That non-permitted ionic configurations (species) have an activity of zero. 2: That the sum of all activities is unity. 3: That the sum of all activities of species containing a particular ion in a particular site is the site occupancy of that ion. The method is computationally simple and yields activity values that satisfy the distribution of species equations of an ideal complex solution model. Standard state properties for annite and Margules solution parameters for biotite are determined using mathematical programming techniques on published experimental and natural assemblage data. Published volume data indicate that Fe-Mg mixing in biotite is ideal. The data permit the calculation of four Margules parameters (MgTi, FeTi, MgAl, FeAl). The differences MgTi - FeTi and MgAl - FeAl are similar to those found by previous workers but the treatment of the data suggests moderately large individual values for the Margules parameters (up to 75 kJ/mol). Using these activity models the SGAM thermobarometer is applied to several sets of published analyses which show that this calibration offers distinct improvements over previous calibrations. Pressures determined using the new calibration are consistent with other barometers and the aluminosilicate polymorph present. In addition, several data sets show field gradients, particularly in P, not previously recognized and which agree with field observations. The SGAM barometer applied to the analytical data from the Quesnel Lake area yields pressures and temperatures that are consistent with the mapped isograds. The pressure and temperature gradients indicate that the final setting of the thermobarome-ter was diachronous across the area and during the early stages of F₃ folding. Hot rocks in cores of anticlines 'set' at later times and at shallower depths than cooler rocks in adjacent synclines. Tight spacing of isograds is more consistent with post-metamorphic folding than with high thermal gradients.
Science, Faculty of
Earth, Ocean and Atmospheric Sciences, Department of
Graduate

Petrologic, structural and age investigations of the Lolotoi Complex of East Timor indicate that it is part of a group of thin metamorphic klippen found throughout the region that were detached from the Banda forearc and accreted to the NW Australian continental margin during Late Miocene to Present arc-continent collision. Metamorphic rock types are dominated by (in order of greatest to least abundance), greenschist, graphitic phyllite, quartz-mica schist, amphibolite and pelitic schist. Mineral, whole rock, and trace element geochemical analyses of metabasites indicate that protolith compositions are consistent with tholeiitic basalt and basaltic andesite with mixed MORB and oceanic arc affinities. Metapelitic schist compositions are consistent with mafic to intermediate oceanic to continental arc provenance. Geothermobarometric calculations show peak metamorphic temperatures in pelitic rocks range from 530°C to 610°C for garnet-biotite pairs and peak pressures of 5 to 8 kbar for garnet-aluminosilicate-quartz-plagioclase assemblages. Analyses of amphibole in amphibolites yield temperatures of 550°C to 650°C and pressures of 6 to 7 kbar. Lu-Hf analyses performed on garnet samples from two massifs in East Timor yielded four ages with a mean of 45.36 ± 0.63 Ma, which is interpreted to represent the approximate age of peak metamorphism. Detrital zircons from one amphibolite sample in East Timor yields a bimodal U-Pb age distribution of 560 Ma and 80 Ma, indicating deposition occurred after the 80 Ma closure of the zircon grains. The sequence of deformation as indicated by field measurements is similar to that reported from other klippen throughout the Timor region. Contact relationships with adjacent units indicate that the metamorphic terrane is in thrust contact with underlying Gondwana Sequence rocks. Overlying the metamorphic rocks are Asian affinity volcanic and sedimentary cover units found mostly in normal fault contact on the edges of Lolotoi Complex klippen. Geochemical, age, petrological and structural data imply the Lolotoi Complex formed part of the eastern Great Indonesian arc, which began to collapse in the Eocene, was incorporated into the Banda arc in the Miocene, and accreted to the Austrailian continental margin from Pliocene to Present.

Thèse (D.R.Min.) -- Université du Québec à Chicoutimi, 1992.
These présentée en collaboration de l'Université du Québec à Chicoutimi et Institute of geochemistry, Academia Sinica. CaQCU Document électronique également accessible en format PDF. CaQCU

El volcanismo oligo-mioceno del Sulcis, SO de Cerdeña (Italia) presenta la particularidad de contener rocas peralcalinas y alcalinas de un ambiente geodinámico anorogénico posterior a la actividad orogénica relacionada con la subducción de una placa oceánica durante la migración antihoraria del bloque Sardo-Corso y la sincrónica creación de la cuenca Liguro-Provençal. La geocronología 40Ar/39Ar (feldespatos), marca el final de la actividad magmática calcoalcalina a 16.18±0.04 Ma (Burdigaliense) y el inicio del episodio anorogénico a 15.96±0.04 Ma (Langhiniense inferior). La duración del evento peralcalino fue de 350 ka según edades U-Pb en zircones. El volcanismo del Sulcis está relacionado con una caldera elíptica de 30x20 km con su eje mayor de dirección NE-SO, de tipo depresión volcano-tectónica de apertura gradual, relacionada con esfuerzos transcurrentes senestrales a través de lineamientos tectónicos regionales paralelos a la Fosa Sarda. Un modelamiento geológico 3D en el Entroterra Sulcitano permitió calcular 21 km3 de flujos ignimbríticos eruptados en un período de ~1.5 Ma. Se estima que el volumen eruptivo en todo el SO de Cerdeña se aproxima a 100 km3, constituyendo una de las mayores calderas del Mediterráneo Occidental. Importantes mineralizaciones de Mn (Fe-Ba) de origen volcánico-hidrotermal están controladas estructuralmente por la intersección de fallas radiales con el sistema de anillos de la caldera. Una cartografía detallada en San Pietro, Sant’Antioco y el Entroterra Sulcitano, permitió dividir una secuencia de ~1300 m de rocas ígneas dómicas, lávicas y piroclásticas en 7 series volcánicas con 25 unidades y 42 subunidades compuestas por andesitas, traquiandesitas, riolitas, comenditas y pantelleritas. Importantes evidencias de vulcanismo submarino se presentan en la Isla de San Pietro en un ambiente marcado por actividad magmático-hidrotermal al final de la serie riolítica media (MRH) y la serie comendítica inferior (LCO). Esta actividad tectono-magmático-hidrotermal es más importante en San Pietro, entre Macchione y Cala Fico, a lo largo de una falla E-O radial a la caldera, con un depósito tipo SEDEX proximal intercalado entre las unidades de Montagna di Capo Rosso y Matzaccara, compuesto por óxidos de Mn, jaspes y ocres que corresponden con el primer sistema mineralizado genéticamente relacionado con el colapso de la caldera, cambio en las condiciones locales del nivel del mar, reactivación de fallas que limitan horst y grabens y circulación de fluidos de origen magmáticos y marino. Este sistema hidrotermal está activo en la parte superior de la serie MRH durante ~10 ka. Un segundo sistema hidrotermal se produce con la erupción de las primeras unidades comendíticas de la serie LCO, también en un ambiente con influencia marina y mineralizaciones de Mn-Fe de tipo hidrotermal-epigenético que ocurren ~220 ka después del primer evento. Mineralizaciones menores de Mn suceden durante la erupción de la unidad Monte Ulmus en San Pietro y Sant’Antioco, y en las unidades Punta Mingosa y Post-Calasetta, principalmente asociados con zonas de blisters donde precipitan óxidos de Fe y se produce argilización y silicificación intensa. Los blisters en las unidades superiores de la secuencia indican un período de desgasificación importante de la cámara magmática en los estadios terminales del magmatismo del Sulcis. La geoquímica isotópica de elementos radiogénicos (Sr-Nd-Pb) muestra que la cuña mantélica implicada en la generación de rocas andesíticas y riolíticas del ambiente orogénico tienen mezcla de componentes DMM+EM1, mientras que los magmas peralcalinos y los previos (MRH) presentan una fuerte señal del componente EM2. El estudio comparado de elementos traza y radiogénicos (87Sr/86Sr vs. Zr/Nb) muestra un enriquecimiento en la isotopía de Sr a medida que los magmas peralcalinos evolucionan por cristalización fraccionada, esto permite trazar los aportes de magmas menos evolucionados que llegan sucesivamente, mostrando una señal mantélica de mayor profundidad, desencadenado las erupciones.
The Oligo-Miocene volcanism from SW Sardinia (Italy) presents peralkaline and alkaline rocks of an anorogenic geodynamic environment erupted after the orogenic magmatism. 40Ar/39Ar geochronology marks the end of the calc-alkaline magmatic activity at 16.18±0.04 Ma and the beginning of the anorogenic episode at 15.96±0.04 Ma. The Sulcis volcanism related to a 30x20 km piecemeal elliptical caldera with a NE-SW direction major axis was formed in a sinistral transtensional regimen. A 3D modeling on Sulcis Mainland provides 21 km3 of ignimbrite flows erupted in ~1.5 Ma (in all likelihood 100 km3 for the whole caldera). Significant Mn mineralizations of volcanic-hydrothermal origin are controlled by the intersection of radial faults with the caldera ring system. Detailed mapping in Sulcis region shows a sequence of ~1300 m (domes, lavas and pyroclastic flows) in 7 volcanic series (25 units and 42 subunits from andesites to peralkaline rhyolites). On San Pietro Island, evidence of submarine volcanism is shown in an environment marked by magmatic-hydrothermal activity at the end of the middle rhyolitic (MRH) and the lower comenditic (LCO) series. This tectonic-magmatic-hydrothermal activity is more important between Macchione and Cala Fico area, along a radial E-W fault, with respect to the caldera ring. In this context, there is a vent-proximal Sedex type deposit with Mn oxides, jaspers and ochres. This deposit corresponds to the first mineralized system genetically related to the caldera collapse, along with changes in sea level local conditions, faults reactivation limiting horst and grabens, and fluids circulation of magmatic and marine origin. This hydrothermal system is active on top of the MRH series for ~10 ka. A second event is produced by the eruption of the first comenditic units in the LCO series, also in a marine-influenced and hydrothermal-epigenetic Mn-Fe mineralization environment, ~220 ka after the first event. Minor hydrothermal mineralizations occur during the eruption of the Monte Ulmus unit in San Pietro and Sant'Antioco. The Sr-Nd-Pb isotopes show a magma generation starting by DMM+EM1 that evolves to an EM2 component at MRH and peralkaline rocks. The 87Sr/86Sr vs. Zr/Nb relation shows radiogenic Sr isotopes enrichment as peralkaline magmas evolve by fractional crystallization.

La premiere partie traite de l'analyse structurale de cete couverture. La carte en courbe de differenciation est discutee : le contenant et le contenu sont analyses; la stabilite de l'information sturcturale et de l'image cartographique lors d'une reduction de la maille ou de l'echelle est abordee. Ainsi la differenciation pedologique est marquee par le transfert de l'argile. Le profil d'eluviation-illuviation est d'autant plus lateral et net que la pente est forte; l'hydrochimie se declenche a l'aval quand la porosite est colmatee par l'argile illuviale. En revanche, la distribution des horizons organiques superficiels est regie par l'erosion. La stabilite de cette couverture epaisse sous des conditions climatiques mediterraneennes agressives est reliee au developpement d'une importante microagregation. L'etude de la stabilite et de la constitution des differentes classes d'agregats montre le role du fer amorphe et de sa liaison avec les acides humiques dans le developpement et la perennite de cette microagregation. Le fer provient en grande partie de l'alteration des chlorites qui aboutit en meme temps a la formation de mineraux a comportement intergrade dont les cales interfoliaires sont essentiellement ferriferes. Deux voies d'approche ont ete suivies pour l'etude de ces mineraux : -l'extraction chimique des amorphes externes et internes et le suivi du comportement a la diffraction rx des argiles traitees : - l'etude de l'alteration des chlorites ferriferes par voie microanalytique

Deux domaines tectono-métamorphiques sont distingués dans le Sud Ouest du Congo : la chaîne précambrienne du Mayombe qui s'étend le long du littoral atlantique depuis le Gabon jusqu'au Nord de l'Angola ; le socle polycyclique de Guéna. Les séries constituant la chaîne du Mayombe sont affectées par deux phases tectoniques qui correspondent aux déformations pan-africaines caractérisées par une schistosité I de direction N130 et pendage SW, une schistosité II, subverticale, sensiblement de même direction qui replisse la première. Elles ont été affectées par un métamorphisme de moyenne pression dont l'intensité diminue progressivement d'Ouest en Est depuis le faciès amphibolite jusqu'à l'anchizone. L'ensemble des formations de la chaîne du Mayombe résulterait de l'ouverture, puis de la fermeture d'un bassin océanique s'achevant par la colision entre un socle occidental, éburnéen (socle de Guéna) et deux socles archéens orientaux (le Chaillu au Nord et le Kasaï au Sud)

Reconstitution chronologique de l'alteration hydrothermale du granite de beauvoir, apex granitique d'echassieres (massif central), a partir des donnees petrographiques, mineralogiques, et datations k-ar. L'etude des isotopes stables oxygene-hydrogene et des inclusions fluides a permis d'etablir un modele thermodynamique sur le refroidissement du magma granitique. Dans les micashistes ces phenomenes hydrothermaux se superposent aux alterations associees au stockwerk a wolframite de l'hypothetique granite de la bosse

碩士
國立東華大學
自然資源與環境學系
101
The Yuli belt of the eastern Taiwan is generally divided into two parts: tectonic (or exotic) blocks and in-situ schists. The former consists mainly of serpentinite and metabasite, whereas the latter is represented by metapelite and metapsammite with subordinate greenschist. The contacts between these two different litho-units tend to contain newly-recrystallized minerals that indicate a possible origin of metasomatism. In the in-situ metapelites, this study found a new type of two-stage garnet zoning with a sharp break in between. Zone I garnet (core) is overgrown by Zone II garnet (mantle). Compositional patterns of Zone I and II imply two stages of garnet prograde growth (M1 and M2). Chlorite commonly replaces Zone II garnet in the outer-most rim, indicating a late-stage retrograde metamorphism (M3). Some amphiboles in the metabasite show compositional zoning with a magnesiohornblende core and an actinolite rim; rare barroisitic compositions are also detected in the core of some samples. The in-situ schist also contains tourmaline, which commonly shows zoning. Plots of tourmaline core compositions on Al-Fe(tot)-Mg and Ca-Fe(tot)-Mg diagrams show great variation and suggest that the tourmaline grains were originated from different protolith sources or geologic environments (cf. Henry et al., 1992), whereas the rim compositions are consistent indicating a metamorphic growth. The metamorphic temperature of M1 was estimated at 560 ± 30˚C with the garnet-ilmenite thermometer (Pownceby et al., 1987) assuming equilibrium. The M2 pressure was estimated at 10-12 kbar on the basis of phengite Si content isopleths with temperature constraint at 560 ± 50˚C. A schematic clockwise P-T path is proposed for the in-situ schist, implying an evolution from subduction to collision for the Yuli belt metamorphic rocks (cf. Beyssac et al., 2008).

碩士
國立東華大學
自然資源與環境學系
103
Tectonic blocks and sheets of mafic-ultramafic rocks are distributed discontinuously in the young Yuli metamorphic belt of Taiwan and are regarded as pieces of a dismember ophiolite. The blocks include rare omphacite metagabbros and garnet-epidote-blueschists in the Wanjung and Juisui (Tamayen) areas, respectively. Such high-pressure (HP) mineral assemblages have been attributed to a mid Miocene subduction event. However, the surrounding psammitic, pelitic and chloritic schists are the dominant greenschist-facies lithologies of the Yuli belt. In the Chinshuichi area, tectonic blocks are enclosed in garnet-bearing metapelites, suggesting more elevated pressures. In this area, I recently discovered meta-plagiogranite containing the assemblage glaucophane + omphacite (XJd up to 0.39) + rutile + quartz, indicating conditions near 14 kbar/550 °C. New equilibrium-phase modeling of a garnet-paragonite-metapelite and compositional isopleths for peak assemblage minerals garnet and phengite (Si = 3.33-3.37 pfu) indicate metamorphic conditions of 15.5-17 kbar/535-550 °C. These P-T estimates are higher than previously reported in the Yuli belt and suggest that both tectonic blocks and host metapelites underwent HP metamorphism. The juxtaposition of tectonic blocks and metapelites apparently occurred during the formation of a subduction-accretionary complex, followed by exhumation facilitated by a collisional event. These findings imply that HP metamorphism was not limited to tectonic blocks as previously thought, and played a significant role attending orogenesis in eastern Taiwan.

M.Sc.
The Salt River deposit is a poly-metallic base metal deposit with a Zn-Cu-Pb metal content that occurs southwest of the town of Kakamas within the Northern Cape Province, South Africa. The Salt River deposit occurs within the Geelvloer Formation of the Bushmanland Subprovince of the Proterozoic Namaqua Metamorphic Province (NMP). This study constitutes the first detailed study of the host rock succession to the Salt River deposit, by investigating the lithostratigraphy, petrography geochemistry and geochronology. During the course of the study, various styles of wall-rock alteration were identified and investigated to determine their effect on the host rock succession. A further aim of this study was to classify the Salt River deposit and compare it to neighboring deposits occurring in the NMP. Geochronological studies were undertaken to define the age of mineralization. Detailed logging of exploration diamond drill core combined with petrographic investigation was used to define thirteen distinct lithotypes. The stratigraphy is dominated by felsic grey gneisses and mafic amphibolites, minor calc-silicate rocks, granitic augen gneisses, pegmatites and two lithologies that represent the metamorphosed equivalents of hydrothermally-altered host rock. Lithostratigraphic investigations yielded a rather uniform succession containing four distinct marker beds defined by their common occurrence and ease of correlation across various boreholes.

Thesis submitted for the degree MAGISTER SCIENTIAE to the Faculty of Science, Department of Geology, University of the Witwatersrand, Johannesburg
The Koperberg Suite intrusion at Jubilee, Namaqualand varies in composition from anorthosite, through mica diorite to pyroxene leucodiorite and pyroxene diorite. Detailed mapping and petrological studies of these rocks indicate that they were sequentially emplaced into the Concordia Granite country rocks, and that each of the rock types represent discrete magmatic events. The sequence of intrusion is from more acidic to more basic. Whole-rock geochemical analyses indicate that these rocks represent cumulates involving variable proportions of plagioclase, orthopyroxene, mica, quartz, oxides and immiscible sulphides. These cumulate phases intruded into the Concordia Granite at the time of peak of metamorphism and deformation in the Okiep Copper District, i.e, about 1030Ma ago, At the time of intrusion, the country rocks were under going partial melting under high grade (granulite facies) metamorphic conditions. and granitic anatects were present in the crust. Hybridisation of basic magma and granitic melts occurred within the shear zones along which the basic magmas ascended. The En content of orthopyroxene in the Koperberg Suite exceeds the An content of plagioclase. This is atypical of basic intrusions and is a consequence of this mixing. Mixing calculations based on the initial 87Sr/86Srratio (Ru) of the Jubilee samples at 1030Ma, imply high levels of assimilation (as much as 80% assimilation in the case of anorthositic rocks) between a granitic component, similar in composition to the Nababeep Gneiss and a mantle-derived basic magma. Sulphide mineralisation was initiated by the assimilation process, which caused the separation of immiscible sulphides from the hybridised magma. Subsequent oxidation of the original sulphide assemblage produced bornite, chalcopyrite and Ti-poor magnetite.
Andrew Chakane 2019

The Madurai Block is the largest granulite block in Southern Granulite Terrain which lies between Palghat-Cauvary shear zone in the North and Achankovil shear zone in the South. This terrain underwent extreme crustal metamorphism under ultrahigh-temperature metamorphic conditions which provides vital information about the tectonic process of the lower crust. Ultrahigh temperature metamorphism was defined by Harley (1998b) as a subclass of granulite facies metamorphism of crustal rocks in which peak temperature exceeds 900°C at moderate pressures (7-13 kbar) in the deep crust. However, considering the lacunae about the present understanding of ultrahigh temperature metamorphism, the study attempts to identify the heat source and role of lower crustal fluids in high temperature metamorphism. To understand the role of lower crustal fluids, a case study on migmatised metapelites from the Kodaikanal region was done where the metapelites have undergone UHT metamorphism. In-situ electron microprobe Th-U-Pb isochron (CHIME) dating of monazites in a leucosome and surrounding silica saturated and silica under saturated restite from the same outcrop indicate three principal ages which can be linked in with the evolutionary history of these rocks. The monazite grains in leucosome sample show alteration along the rims. These altered rims are experimentally replicated in a monazite-leucosome experiment at 800°C and 200MPa. This experiment, coupled with earlier published monazite-fluid experiments involving high pH alkali-bearing fluids at high P-T, helps to confirm the idea that alkali-bearing fluids, in the melt and along grain boundaries during crystallization, were responsible for the formation of the altered monazite grain rims via the process of coupled dissolution-reprecipitation. Lower crustal fluids during migmatization and high temperature metamorphism from leucosome monazites signify the need for a more precise texturally-controlled geochronological determination. Considering the possible heat source of high temperature metamorphism, the role of associated rocks of charnockites/granites and ultramafics was studied from Kodaikanal and Ganguvarrpatti. The results indicate that both charnockites and granites are not the heat source of high temperature metamorphism. However, to recognize the ultramafic as the potential heat source the sapphirine-bearing high Mg-Opx bearing rock was studied from Kambam town. The sapphirine–cordierite intergrowth pods are characterized by unique texture and peraluminous sapphirine composition suggesting that these domains could represent cryptic pathways through which aluminous melts migrated. The mineral phase equilibria considerations suggest that such peraluminous melts interacted with Mg-rich orthopyroxene in the host granulite at 1025°C and 8 kbar, with subsequent isobaric cooling. The underplated mafic magma (T>1000°C) is suggested as a possible mechanism that provided the heat source for partial melting of lower crust and the UHT metamorphism. Moreover, field evidence of metapelite in direct contact with an ultramafic body was observed resulting into granulite grade metamorphism. The other evidence of ultramafic magma as heat source is though the mineral chemistry and geochemical modeling of the studied ultramafic rocks.