HIGH-TEMPERATURE METASOMATISM IN ULTRAMAFIC GRANULITES OF HIGHLAND COMPLEX, SRI LANKA
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Concordant ultramafic rocks are exposed in the lower crustal granulites of Sri Lanka. The ultramafic rocks at Rupaha had emplaced early. They had been subjected to deformation and granulite facies metamorphism at 850 o C at 9 kbar during the PanAfrican tectono-thermal episode. The results of thermometry of ultramafic rocks are consistent with those of geo-thermobarometry obtained from surrounding granulites. Fluids circulating in the deep crust had caused the formation of phlogopite blackwalls. Structure, texture and mineralogy of the blackwalls suggest that the K-metasomatism had taken place contemporaneous to the granulite facies metamorphism. The metasomatic reactions had started due to infiltration of K2O and SiO2 between ultramafic rocks and surrounding gneisses and diffusion of these elements between two wall rocks respectively. Carbonation and hydration had occurred in the blackwall rocks at the upper level of the crust on cooling together with surrounding rocks as indicated by textures of a partial retrogression to tremolite and dolomite, which formed throughKeywords:
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The Bohemian Massif (BM), the easternmost part of Variscan chain in Europe, contains both autochthonous and allochthonous terrains. Thin skinned units (nappes) characterize parts of Moldanubian, and Saxothuringian and cover the deep structure of BM. The occurrences of granulites in Moldanubian (Gfohl unit) and the crustal (granulite) xenoliths in alkaline basalts of North Bohemian Cenozoic province indicate the presence of two principally different lower crust sections in and under the nappes. The southern part of the Bohemian Massif (exposed at the surface) is characterized by the presence of high-P high-T “ga–ky granulites” (accompanied by garnet peridotites) whereas the northern part, containing pyroxene granulites (charnockites) with high-T, low-P paths, is accompanied by upper mantle four-phase lherzolites. Both granulites are compositionally close to granites in respect to Ab–Or–Oz relations and differ in respect to trace element compositions. Garnet–kyanite granulites have trace-element abundances with granite features: high LILE contents, LREE-enriched patterns, negative Eu anomaly (Fig. 1), but low Th and U and varied Zr abundances. The contents of siderophile and transitional elements are slightly higher than corresponding contents in granites. Positive correlation of Zr with Th and U, and negative correlation of Zr with LILE elements, e.g., Rb suggests that melting or removal of zircon played a major role in generating the granulite trace element signature. North Bohemian charnockites that are LILE poor with Eu positive anomalies resemble “shield granulites” (Fig. 2). Southern Bohemian granulite bodies contain blocks and xenoliths of ultramafic rocks not related to metamorphic structures of granulites. Xenoliths occur in both ga–ky (not retrogressed) and ga–bi (retrogressed) rocks. Ultramafic rocks were incorporated into granulites in the solid state and their mineral equilibria indicate origin in the lithospheric as well as asthenospheric mantle and consequent fast exhumation. The interval of time of protolith crystallization of granulites (i.e., 370–340 Ma) is narrow and indicates a “granulite event” in the early Variscan. The event is contemporaneous with the emplacement of “igneous rocks” that share granulite facies features (e.g., opx–cpx parageneses), have strong geochemical mantle signatures and are part of major batholiths (durbachite types) in Moldanubian terrane (Tabor, Jihlava, Weinsberg, Rastenberg). Differences in metamorphic grade of granulites and surrounding gneisses, composition, and presence of ultramafic xenoliths suggest that ga– ky granulites are pristine dehydration melts that may have formed at collisional environment at the expense of crustal material of lower plate. These melts have crystallized with ga and ky as a near liquidus phases or as a product of dehydration incongruent melting. On the rise from the lower plate the melts captured ultramafic rocks and reaction of ga → opx + plg has taken place. The intrusions of ga–ky granulites stopped in the middle crust (amphibolite facies). Structure of granulite bodies and metamorphic rock textures relate to the retrogressive changes, postdate ga–ky and px-bearing parageneses and represent post-emplacement features. The granulites (charnockites) of North Bohemia are distinctly different and may represent fragments of Baltica or may be remnants of such microcontinents as Armorica, Perunica, or Avalonia.
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Abstract The Catalina Schist of southern California is a subduction zone metamorphic terrane. It consists of three tectonic units of amphibolite‐, high‐ P greenschist‐ and blueschist‐facies rocks that are structurally juxtaposed across faults, forming an apparent inverted metamorphic gradient. Migmatitic and non‐migmatitic metabasite blocks surrounded by a meta‐ultramafic matrix comprise the upper part of the Catalina amphibolite unit. Fluid‐rock interaction at high‐ P , high‐ T conditions caused partial melting of migmatitic blocks, metasomatic exchange between metabasite blocks and ultramafic rocks, infiltration of silica into ultramafic rocks, and loss of an albitic component from nonmigmatitic, clinopyroxene‐bearing metabasite blocks. Partial melting took place at an estimated P =˜8–11 kbar and T =˜640–750°C at high H 2 O activity. The melting reaction probably involved plagioclase + quartz. Trondhjemitic melts were produced and are preserved as leucocratic regions in migmatitic blocks and as pegmatitic dikes that cut ultramafic rocks. The metasomatic and melting processes reflected in these rocks could be analogous to those proposed for fluid and melt transfer of components from a subducting slab to the mantle wedge. Aqueous fluids rather than melts seem to have accomplished the bulk of mass transfer within the mafic and ultramafic complex.
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Research Article| December 01, 1968 Regional Metamorphism, Metasomatism, and Partial Fusion in the Northwestern Part of the Okanogan Range, Washington JAMES W HAWKINS, JR. JAMES W HAWKINS, JR. Geological Research Division, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California Search for other works by this author on: GSW Google Scholar Author and Article Information JAMES W HAWKINS, JR. Geological Research Division, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California Publisher: Geological Society of America Received: 05 Dec 1967 Revision Received: 01 Jul 1968 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1968, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1968) 79 (12): 1785–1820. https://doi.org/10.1130/0016-7606(1968)79[1785:RMMAPF]2.0.CO;2 Article history Received: 05 Dec 1967 Revision Received: 01 Jul 1968 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation JAMES W HAWKINS; Regional Metamorphism, Metasomatism, and Partial Fusion in the Northwestern Part of the Okanogan Range, Washington. GSA Bulletin 1968;; 79 (12): 1785–1820. doi: https://doi.org/10.1130/0016-7606(1968)79[1785:RMMAPF]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The northwestern part of the Okanogan Range, Washington, (in the vicinity of 49°00 ′N., 120°00′W.) comprises regionally metamorphosed rocks, largely of supracrustal origin, plus igneous rocks ranging in composition from quartz diorite to quartz monzonite.Metamorphic rocks include quartz dioritic to trondhjemitic orthogneisses, biotitic and hornblendic schists, gneisses and hornfelses, amphibolites, and calc-silicate gneisses. Mineral assemblages indicate that the region was metamorphosed more or less uniformly under conditions equivalent to the sillimanite-cordierite-orthoclase-almandine subfacies (cordierite amphibolite facies) with possible local transition to granulite facies. The presence of andalusite plus sillimanite indicates metamorphism of the Abukuma type (andalusite-sillimanite facies series) in contrast to the kyanite-sillimanite facies series in the Northern Cascade Range to the west. A single cycle of synkinematic metamorphism followed by a phase or separate cycle of static recrystallization in essentially the same P-T field is shown by rock textures and mineral assemblages. The parent material of the metamorphic rocks included mafic flows, sills and dikes, intermediate composition volcanic rocks and graywackes, minor amounts of carbonate-rich and pelitic sedimentary rocks plus an indeterminate amount of leucocratic igneous rock. Prior to metamorphism the average composition of the parent material was approximately tonalitic.The igneous rocks include intrusive granodioritic and quartz monzonitic rocks (average composition is granodiorite) of the Cathedral batholith (Daly, 1912) with an apparent age of 94.0 ± 2.8 m.y. (K/Ar date on biotite) and older trondhjemitic to leucogranodioritic rocks. The older igneous series comprises partly gneissose syntectonic intrusive rocks, and directionless late to post-tectonic rocks which lack intrusive contacts. The latter are interpreted as anatectites formed by (partial) fusion of rocks of quartz dioritic to trondhjemitic composition during regional metamorphism. Field and petrographic data indicate that the anatectites have not been deformed. The average chemical composition of the older igneous series is trondhjemitic. A small layered gabbroic pluton, characterized by iron-rich olivine and pyroxene, has intruded the metamorphic series. Its age relative to the Cathedral batholith is not known.The metamorphism was pre-late Cretaceous and, by analogy with surrounding areas, probably was pre-mid Jurassic but post-Paleozoic. The age of the eugeosynclinal rocks, which were parent materials for the metamorphic rocks, is unknown, but a mid- to late-Paleozoic age is suggested by comparison with rocks of known age in adjacent areas. These eugeosynclinal rocks were definitely sialic, their average composition being tonalitic. A comparison between the average chemical composition of the metamorphic rocks and syn-metamorphic plutons and the average composition of eugeosynclinal rocks of the Pacific rim, such as graywackes, suggests that the rocks of the Okanogan Range may have been formed by nearly isochemical metamorphism of eugeosynclinal sedimentary and volcanic rocks. This content is PDF only. Please click on the PDF icon to access. 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ABSTRACT The northern Dabie terrane consists of a variety of metamorphic rocks with minor mafic‐ultramafic blocks, and abundant Jurassic‐Cretaceous granitic plutons. The metamorphic rocks include orthogneisses, amphibolite, migmatitic gneiss with minor granulite and metasediments; no eclogite or other high‐pressure metamorphic rocks have been found. Granulites of various compositions occur either as lenses, blocks or layers within clinopyroxene‐bearing amphibolite or gneiss. The palaeosomes of most migmatitic gneisses contain clinopyroxene; melanosomes and leucosomes are intimately intermingled, tightly folded and may have formed in situ. The granulites formed at about 800–830 °C and 10–14 kbar and display near‐isothermal decompression P–T paths that may have resulted from crust thickened by collision. Plagioclase‐amphibole coronae around garnets and matrix PI + Hbl assemblages from mafic and ultramafic granulites formed at about 750–800 °C. Partial replacement of clinopyroxene by amphibole in gneiss marks amphibolite facies retrograde metamorphism. Amphibolite facies orthogneisses and interlayered amphibolites formed at 680–750 °C and c. 6 kbar. Formation of oligoclase + orthoclase antiperthite after plagioclase took place in migmatitic gneisses at T ≤ 490°C in response to a final stage of retrograde recrystallization. These P–T estimates indicate that the northern Dabie metamorphic granulite‐amphibolite facies terrane formed in a metamorphic field gradient of 20–35 °C km ‐1 at intermediate to low pressures, and may represent the Sino‐Korean hangingwall during Triassic subduction for formation of the ultrahigh‐ and high‐P units to the south. Post‐collisional intrusion of a mafic‐ultramafic cumulate complex occurred due to breakoff of the subducting slab.
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The Grove Mountains, East Antarctica, consist of granulite-facies high-grade metamorphic rocks and some granitoids. Among them, the metamorphic rocks are dominated by pale and dark orthopyroxene-bearing felsic gneiss, with minor mafic granulite, metasedimentary rock and occasionally scapolite-bearing calc-silicate rock. All metamorphic rocks exhibit an equilibrium texture, but exsolution lamellae of orthopyroxene (pigeonite) occur in all clinopyroxenes in mafic granulites. A peak metamorphic temperature of c. 850℃ was obtained from the reintegrated compositions of exsolved clinopyroxene, and a pressure of 0. 61 -0. 67 GPa from garnet-orthopyroxene-plagioclase-quartz geobarometer. The preservation of igneous augite megacrysts in mafic granulites suggests a single episode of Pan-African granulite-facies metamorphism developed in the Grove Mountains. The rocks later underwent a slow cooling process, which is attributed to the magmatic underplating of the lower crust.
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Granulite xenoliths are found in the early Mesozoic diorite intrusions from Chifeng and Ningcheng areas, eastern Inner Mongolia. The granulites are granoblastic and weakly gneissic with mineral assemblage of hypersthene, diopside, plagioclase and minor biotite, amphibole and ilmenite. Some samples contain the intergrowth composed of labradorite and vermicular hypersthene, and some coarse-grained plagioclases of andesine and labradorite composition occasionally develop bytownite rims with vermicular hypersthene, indicating a possible presence of garnet. Presence of blastogabbroic texture and hypersthene with diopside exsolution lamellae in some samples suggests that the protolith of the granulite is norite or gabbro. On the basis of metamorphic research and thermobaric calculation, the evolution of the granulite xenoliths is summarized into the following stages: (1) Isobaric cooling of underplated noritic or gabbroic magma in the lower crust led to the formation of probable garnet-bearing medium-high pressure granulite. (2) These higher pressure granulites were adiabatically uplifted to upper crust by dioritic magma and transformed to low pressure two-pyroxene granulite during an isothermal decompression. (3) The two-pyroxene granulite underwent retrograde metamorphism of different degrees during an isobaric cooling process as a result of crystallization and cooling of the dioritic magma. The pyroxenite-dominated cumulates and the medium-high pressure granulites may have rejuvenated the lower crust during the early Mesozoic.
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The Lišov Granulite Massif differs from neighbouring granulite bodies in the Moldanubian Zone of southern Bohemia (Czech Republic) in including a higher proportion of intermediate–mafic and orthopyroxene-bearing rocks, associated with spinel peridotites but lacking eclogites. In addition to dominantly felsic garnet granulites, other major rock types include quartz dioritic two-pyroxene granulites, tonalitic granulites and charnockites. Minor bodies of high-pressure layered gabbroic garnet granulites and spinel peridotites represent tectonically incorporated foreign elements. The protoliths of the mafic–intermediate granulites (quartz-dioritic and tonalitic) crystallized ∼360–370 Ma ago, as indicated by laser ablation inductively coupled plasma mass spectrometry U–Pb ages of abundant zircons with well-preserved magmatic zoning. Strongly metamorphically recrystallized zircons give ages of 330–340 Ma, similar to those of other Moldanubian granulites. For the overwhelming majority of the Lišov granulites peak metamorphic conditions probably did not exceed 800–900°C at 4–5 kbar; the equilibration temperature of the pyroxene granulites was 670–770°C. This is in sharp contrast to conditions of adjacent contemporaneous Moldanubian granulites, which are characterized by a distinct HP–HT signature. The mafic–intermediate Lišov granulites are thought to have originated during Viséan metamorphic overprinting of metaluminous, medium-K calc-alkaline plutonic rocks that formed the mid-crustal root of a Late Devonian magmatic arc. The protolith resembled contemporaneous calc-alkaline intrusions in the European Variscan Belt.
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A suite of basic granulites including garnet-plagioclase-hypersthene granulite, hypersthene-biotite-garnet granulite, garnet two-pyroxene-plagioclase granulite and garnet-clinopyroxene granulite are present in the Qingshuiquan Area of the Central Eastern Kunlun Suture Zone, Northwest China. These granulites together with a series of high grade metamorphic rocks, including migmatitized biotite-garnet granulite, biotite-pyroxene granulite, graphite marble, diopside-tremolite-bearing marble, diopside marble, biotite amphibolite and gneisses, and magmatic rocks, such as dunite, harzburgite, troctolite, gabbro, diabase and basalt, define an ophiolitic tectonic melange. The metamorphism of the Qingshuiquan granulites took place under temperature T=760~880℃ and pressure p=8.3~12 kbar, the conditions of high temperature and medium to high pressure, occurring at a depth range of 40~45 km. The SHRIMP U-Pb zircon age of granulite facies metamorphism of the garnet two-pyroxene-plagioclase granulite is 507.7±8.3 Ma (2σ). Ophiolites preserved in the Qingshuiquan Area originated at ~520 Ma, and subducted to a depth range of 40~45 km by ~508 Ma and experienced high temperature and medium to high pressure granulite facies metamorphism, and then were tectonically uplifted and exhumed to the present surface. This study demonstrates that the high grade metamorphic rocks and the mafic-ultramafic magmatic rocks in the Qingshuiquan Area outline an Early to Middle Cambrian tectonic melange, marking a significant plate assembly boundary of the early stage of Paleozoic.
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