Spinel+Quartz association from the Kerala khondalites, southern India: evidence for ultrahigh-temperature metamorphism (original) (raw)
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Journal of Asian Earth Sciences, 2014
Here, we report a natural field example for the coexistence of spinel + quartz as a non-UHT assemblage in spinel-and cordierite-bearing garnet-sillimanite-biotite-graphite gneiss (khondalite) interbedded with orthopyroxene-garnet-biotite bearing intermediate granulites from the Highland Complex (HC) in Sri Lanka. The khondalite contains Zn-rich spinel mainly in four textural assemblages namely: (a) spinel co-existing with tiny quartz (ZnO = 12.67-12.85 wt%), (b) spinel surrounded by sillimanite moates and in intergrowth with skeletal sillimanites (ZnO = 9.03-9.17 wt%), (c) symplectitic spinels at the margin of sillimanite (ZnO = 4.09-4.28 wt%) and (d) spinel co-existing with ilmenite or as isolated grains (ZnO = 7.61-7.97 wt% and Cr 2 O 3 = 5.99-6.27 wt%). Assemblage (a) and (b) occur within garnet while assemblages of (c) and (d) are present within cordierite moates after garnet in the matrix. Pseudosections calculated in the NCKFMASHTMnO system and conventional geothermobarometry suggest that the metamorphic peak conditions attained by the spinel + quartz bearing khondalites and associated intermediate granulites did not exceed T of 900°C and P of 7.5-8.5 kbar. Post-peak evolution was characterized by a stage of nearly-isobaric cooling down to T of 770°C and P of 7.5 kbar, followed by a late stage of isothermal decompression down to P < 6.5 kbar and T of 770°C.
Journal of Geology, 2000
The Kerala Khondalite Belt (KKB) of South India is a pan-African (ca. 600-500 Ma) regional granulite terrain that is composed of charnockites, khondalites, and migmatitic leptinitic gneiss. Peak assemblages in the KKB are characterized by orthopyroxene ϩ garnet ϩ K-feldspar ϩ magnetite ϩ biotite ϩ plagioclase ϩ quartz in felsic to intermediate rocks and by orthopyroxene ϩ cordierite ϩ plagioclase ϩ quartz or garnet ϩ cordierite ϩ sillimanite ϩ feldspars ϩ quartz in semipelites and pelites. Postpeak reaction textures in these assemblages often involve the production of cordierite and are consistent with a predominantly decompressional pressure-temperature history at temperatures in excess of 700ЊC. Pressure-temperature estimates based on garnet-orthopyroxene thermobarometry are in the range of 6.5-7.5 kbar and 860Њ-920ЊC for localities close to and within the Achankovil Shear Zone, a prominent high-grade, high-strain zone on the northeastern margin of the KKB. For this northeastern subarea, peak temperatures of C at 6.5-7.0 kbar are increased to C when retrieval calculations are applied to account for 900Њ ע 20Њ 925Њ ע 25Њ postpeak Fe-Mg exchange. Such near-ultrahigh-temperature conditions, however, are not recorded from the central KKB, where pressure-temperature estimates based on a range of thermobarometers and data sets are 4.8-5.7 kbar and 830Њ-860ЊC. In contrast to earlier studies that suggested that peak pressure-temperature conditions were rather uniform across the whole KKB, these new results, coupled with those in other recent studies, demonstrate that peak metamorphic temperatures varied across the KKB. The highest temperatures attained may reflect the thermal input of dry magmas now exposed as massive charnockites within and to the northeast of the Achankovil Shear Zone, if these are synmetamorphic and similar in age to the main metamorphic event.
Journal of Southeast Asian Earth Sciences
This paper reports several new localities of wollastonite-and scapolite-bearing talc-silicate assemblages from the granulite-facies supracrustal Kerala Khondalite Belt (KKB), southern India. Based on mineralogy, these talc-silicate rocks are classified into four types: Type I, lacking wollastonite and grossular; Type II, wollastonite-bearing but grossular-absent; Type III, wollastoniteand grossular-bearing; and Type IV, dolomitic marbles. Detailed petrographic studies reveal a variety of reaction textures overprinting the polygonal granoblastic peak metamorphic assemblages in these rocks. The Type II talc-silicate rocks preserve reaction textures, including meionite breaking down to anorthite-calcite-quartz, wollastonite breaking down to calcite-quartz and meionite-quartz symplectites after K-feldspar and wollastonite. Type III talc-silicate rocks have porphyroblastic and coronal grossular. Grossular-quartz coronas separating wollastonite and anorthite and the development of grossular within the anorthite-calcite-quartz pseudomorphs of meionite form important retrograde reaction textures in this type. In Type. IV dolomitic marble assemblages, meionite forming in grain boundaries of calcite and feldspars, forsterite rimmed by diopside-dolomite and the formation of grossular in feldspar-rich zones are the important textures. Calculated partial petrogenetic grids in the CaO-AlzO,-SiO&02 system are used to deduce the pressure-temperature fluid evolution of the talc-silicate rocks. The Type II assemblages provide CO, activity estimates of >0.5, with a peak metamorphic temperature of about 790°C. Initial cooling followed by later COZ influx can be deduced from reaction modelling in these talc-silicate rocks. Type III assemblages are characterized by internal fluid buffering throughout their tectonic history. The formation of coronal grossular indicates an initial cooling from peak metamorphic temperatures of about 830°C deduced from vapour-absent meionite and grossular equilibria. Type IV marble assemblages also indicate internal fluid buffering followed by localized CO* influx. Overall, the talc-silicate rocks of the KKB define peak metamorphic temperatures in the range of 790_85o"C, with an internally buffered fluid composition during the peak conditions. Initial cooling was followed by localized carbonic fluid influx that also postdated decompression deduced from other rock types in the KKB.
Journal of Metamorphic Geology, 1997
Migmatitic semipelitic granulites of the Proterozoic Epupa Complex, NW Namibia, underwent ultrahigh-temperature metamorphism as is indicated by the high alumina contents of orthopyroxene (8±11 wt % Al 2 O 3 ) coexisting with garnet. Peak P±T conditions of 970Cand970 C and 970Cand9Á5 kbar are calculated from conventional garnet±orthopyroxene geothermobarometry. Conspicuous reaction textures document a multistage retrograde uplift±cooling path: post-peak decompression initially under still ultrahigh temperatures ($940 C and 8kbar)isrecordedbycoronasofaluminousorthopyroxeneplagioclasearoundgarnet.Continueddecompression(8 kbar) is recorded by coronas of aluminous orthopyroxene plagioclase around garnet. Continued decompression (8kbar)isrecordedbycoronasofaluminousorthopyroxeneplagioclasearoundgarnet.Continueddecompression(6 kbar and 800C)isevidentfromsubsequentlyformedsymplectitesofcordieritelower−aluminaorthopyroxeneandcordieritelower−aluminaorthopyroxenespinel,bothreplacinggarnet.Subsequentregrowthofgarnetandbiotite,mainlyformedattheexpenseofthesymplectiticphases,presumablyreflectsback−reactionswithcrystallizingmeltsduringnear−isobariccoolingtoupperamphibolite−faciesconditions(800 C) is evident from subsequently formed symplectites of cordierite lower-alumina orthopyroxene and cordierite lower-alumina orthopyroxene spinel, both replacing garnet. Subsequent regrowth of garnet and biotite, mainly formed at the expense of the symplectitic phases, presumably reflects back-reactions with crystallizing melts during near-isobaric cooling to upper amphibolite-facies conditions (800C)isevidentfromsubsequentlyformedsymplectitesofcordieritelower−aluminaorthopyroxeneandcordieritelower−aluminaorthopyroxenespinel,bothreplacinggarnet.Subsequentregrowthofgarnetandbiotite,mainlyformedattheexpenseofthesymplectiticphases,presumablyreflectsback−reactionswithcrystallizingmeltsduringnear−isobariccoolingtoupperamphibolite−faciesconditions(660 C and $5 kbar). Rims of low-alumina orthopyroxene around retrograde biotite point to renewed decompression subsequent to cooling. The deduced clockwise retrograde P±T path reflects the thinning and later cooling of former thickened lower crust. Because of the limited structural and geochronological data it remains uncertain whether initial ultrahigh-temperature metamorphism was induced by a collision event or by crustal extension.
Journal of Geodynamics, 2013
The Proterozoic Kerala Khondalite Belt (KKB), southern India preserves a distinct high-grade terrain that is interpreted to have been situated adjacent to Madagascar and Sri Lanka during Gondwana assembly. As such, it has become a major focus for testing models of supercontinent amalgamation and dispersal. The lithounits of KKB have remarkable petrological similarities to the Highland Complex (HC) of Sri Lanka and south-central Madagascar. However, there is no well-constrained petrogenetic model for the KKB that fits explicitly within a supercontinent reconstruction model. We present here results from our ongoing studies on the origin and evolution of K-rich (potassic, where K 2 O/Na 2 O > 1) gneisses of KKB in relation to Proterozoic supercontinent events. Our results show, in a major departure from earlier metasedimentary origin, that potassic gneisses are metamorphosed granitoids. The metagranitoid samples display high K 2 O contents and low Al 2 O 3 /(FeO + MgO + TiO 2) values. They are moderate to strongly peraluminous (ASI values ranging from 1.05 to 1.47) rocks showing mineralogical, petrological, and geochemical characteristics distinctive of the high-K calc-alkaline suites. Typical of igneous suites, the high-K metagranites show minor variation in chemical compositions with most oxides showing negative correlation with SiO 2. Geochemistry illustrates distinctive features of arc-related magmas with LILE (K, Rb, and Th) and LREE enriched patterns and considerable depletion of HSFE (Nb, Zr, and Ti). The high-K metagranites are further characterized by strong negative anomalies of Eu (Eu/Eu* = 0.10-0.44) and Sr, suggesting melting in plagioclase stability field and retention of plagioclase in the residual phase. Petrogenetic discrimination for granitoids, using major and trace elements demonstrates that the high-K metagranites of the KKB formed by partial melting of igneous source in lower-to middle-crust levels. Overall the geochemical features are supportive of origin in relation to a convergent margin setting, possibly in a continental magmatic arc system, which can be connected to the amalgamation and dispersal of continental fragments in a supercontinent event. This study, therefore, provides a lead towards more robust comparisons between the Proterozoic supercontinent events and processes.
Lithos
Garnet-bearing migmatites and associated leucogranites and leucosomes of the Trivandrum Block in the Kerala Khondalite Belt were formed through granulite facies dehydration melting of metasedimentary protoliths. Significant trace element depletions in Cs, Zr, Nb and the compatible transition metal elements V, Cr, Ni, Cu and Zn relative to average post-Archaean shales and model middle and upper crust, recorded in all samples, require their sedimentary protoliths to have been impure sandstones and greywackes, rather than shales. Leucogranites (70-75wt %) and leucosomes (SiO 2 : 68-70wt %), which are uniformly peraluminous and classed as S-type on the basis of their A/NK and ASI relations, form a compositional array that shows strong correlated variations between TiO 2 and Fe 2 O 3 , and TiO 2 or Fe 2 O 3 with Co and Y. These reflect coupled variations in modal garnet and ilmenite and require the presence of up to 15-20 wt% of entrained peritectic garnet in the higher Y and HREE leucosomes. The leucosomes have REE patterns with normalised La/Sm of 10, negative Eu anomalies (Eu* < 0.8) and flat HREE at 6-20 times chrondrite, whereas leucogranites range to much lower HREE (1-5 times chondrite) with higher La/Sm and large positive Eu anomalies (Eu* = 1.4-3.4). Despite broadly similar major element compositions that lie within the granite minimum melt field in terms of felsic components, leucogranite Zr contents are highly variable, ranging down to 4-20 ppm in the lowest HREE and high Eu* cases, resulting in Zr saturation temperatures (544-647ºC) that are lower than any feasible melt. These geochemical features, coupled with covariations between Nb,-Ta and Y-Yb, collectively support petrological and field observations that the leucosomes are mixtures between former melt and entrained peritectic garnet and ilmenite. The leucogranites are the products of melt extraction and migration on metres to several metres lengthscales. Leucogranite Nb-Y, Ta-Yb, Eu* and Sr-Ba relationships demonstrate that their chemical
Journal of Petrology, 1999
A suite of quartz-and corundum-bearing metapelitic granulites, metamorphism occurred in the lower crust (>8 kbar). Reported intruded by layered gabbronorite-pyroxenite-anorthosite at Kon-U-Pb cooling ages of monazite and allanite from a late pegmatite dapalle, Eastern Ghats Belt, preserves a multitude of reaction suggest the UHT metamorphism to be older than 1600 Ma. The textures involving oxide and silicate minerals that attest to several deduced P-T history and the absence of Grenvillian high-grade prograde and retrograde reactions. In the quartz-bearing associations, metamorphism in the study area provide important constraints on the reactions are: (a) biotite + sillimanite + quartz → garnet the configuration of East Gondwana, in particular on the continuation + liquid; (b) garnet + sillimanite → spinel (+ magnetite) + of the Napier-Rayner terrane boundary into the Eastern Ghats quartz; (c) Fe 2 TiO 4 + O 2 → ferrian ilmenite + magnetite; (d) Belt. reversal of reaction (b); (e) Fe 2 O 3 -rich ilmenite + plagioclase + quartz → Fe 2 O 3 -poor ilmenite + garnet + O 2 . Reactions in the corundum-bearing associations are: (f) Fe 2 TiO 4 + FeAl 2 O 4 + Belt, India; Indo-Antarctic terrane assembly O 2 → ferrian ilmenite + Fe 3 O 4 + Al 2 O 3 (in ilmenite); (j) garnet + corundum → spinel + sillimanite. To examine the paragenetic evolution of the metapelitic granulites, a petrogenetic grid for the KFMASH system at high temperatures and pressures, involving *
Geological Magazine, 2009
Sapphirine-bearing Mg–Al granulites from Rajapalaiyam in the southern part of the Madurai Block provide critical evidence for Late Neoproterozoic–Cambrian ultrahigh-temperature (UHT) metamorphism in southern India. Poikiloblastic garnet in quartzo-feldspathic and pelitic granulites contain inclusions of fine-grained subidioblastic to xenoblastic sapphirine associated with quartz, suggesting that the rocks underwent T > 1000°C peak metamorphism. Quartz inclusions in spinel within garnet are also regarded as clear evidence for a UHT condition. Inclusions of orthopyroxene within porphyroblastic garnet in the sapphirine-bearing rocks show the highest Al2O3 content of up to 10.3 wt%, suggesting T = 1050–1070°C and P = 8.5–9.5 kbar. Temperatures estimated from ternary feldspar and other geothermometers (T = 950–1000°C) further support extreme thermal metamorphism in this region. Xenoblastic spinel inclusions in sapphirine coexisting with quartz suggest that the spinel + quartz assembla...
Ultra high temperature-metamorphism and its significance in the Central Indian Tectonic Zone
Lithos, 2006
In the present study from the southern margin of the Central Indian Tectonic Zone, it is demonstrated how the metamorphic P- T path of ultrahigh-temperature granulite terranes can be reconstructed using the metamorphic transition in corundum granulites from early biotite melting to later FMAS solid-solid reaction. The extreme metamorphism in these rocks caused two-stage biotite melting, resulting in initial porphyroblastic garnet 1 and later sapphirine-spinel 1 incongruent solid mineral assemblages. During this process, the leucocratic and melanocratic layers in the corundum granulites evolved from an initial silica-oversaturated to a later silica-undersaturated domain. In the melanocratic layer, this allowed localized concentration of sapphirine-spinel 1 and residual sillimanite 1, producing an extremely restitic assemblage, at the culmination of peak metamorphism, BM 1. BM 1 is constrained at ˜ 1000 °C at relatively deep crustal levels ( P ˜ 9 kbar) from the stability of ferroaugite in a co-metamorphosed Iron Formation granulite. During subsequent metamorphism (BM 2), the reaction path and history in the corundum granulites shifted to the restitic domain allowing reacting sapphirine, spinel 1 and sillimanite to produce coronal garnet 2-corundum assemblage via a FMAS univariant reaction. In the final stages of reaction history, biotite 2-sillimanite 2-spinel 2 assemblage was produced after garnet 2-corundum due to localized melt-crystal interaction. The metamorphic sequence, when interpreted with the help of a newly constructed, qualitative KFMASH petrogenetic grid, reveals successive stages of heating, increasing pressure and cooling around the KFMASH invariant point, [Opx,Crd], which is consistent with a counterclockwise metamorphic P- T path. The near isobaric nature of post-peak cooling (Δ T ˜ 250-300 °C) is also evident from multistage pyroxene exsolution and by the appearance of lamellar and coronal garnets in the Iron Formation granulites. This study provides the first tight constraint for ultrahigh- T metamorphism along a counter clockwise P- T trajectory in the Central Indian Tectonic zone, and has important bearing for terrane correlations in this part of East Gondwanaland. In addition, the new KFMASH grid allows evaluation of metamorphic phase relations in ultrahigh- T, corundum-bearing and corundum-absent aluminous granulites.