Timing Of High-grade Metamorphism Research Articles

Neoarchean arc magmatism and Paleoproterozoic high-pressure granulite-facies metamorphism in the southern Motloutse Complex, eastern Botswana: Implications for the western extension of the Limpopo Complex

We provide the first record of the occurrence of high-pressure garnet-bearing amphibolite and mafic granulite hosted by amphibolites from the Maope area of the southern Motloutse Complex in eastern Botswana. The mineral assemblages of the rocks are garnet + edenite + plagioclase + ilmenite + rutile + quartz (garnet-bearing amphibolite), magnesio-hornblende + orthopyroxene + clinopyroxene + plagioclase + quartz (mafic granulite), and magnesio-hornblende + plagioclase + quartz + ilmenite + epidote + titanite (host amphibolite). The poikiloblastic garnet in the garnet-bearing amphibolite contains inclusions of edenite, ilmenite, quartz, and plagioclase as prograde minerals, while it is mantled by ferro-edenite + plagioclase coronae formed during retrograde metamorphism. The geochemical signatures of the mafic granulite and amphibolite, combined with the enrichment of large-ion lithophile elements and depletion of high-field strength elements (Nb and Ta), indicate subduction-related volcanic arc affinity, whereas the garnet-bearing amphibolite indicates subduction-related volcanic arc or within-plate basalt affinity. The peak metamorphic condition of the garnet-bearing amphibolite was constrained as 850–910 °C and 10.0–10.5 kbar based on phase equilibrium modeling in the Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (NCFMASHTO) system and geothermobarometry. Slightly lower conditions were obtained from the mafic granulite (745–825 °C) and amphibolite (610–750 °C). Zircon grains from the garnet-bearing amphibolite with lower Th/U ratios of 0.02–0.23 yielded a weighted-mean 207Pb/206Pb age of 2020.2 ± 4.9 Ma as the timing of high-grade metamorphism. In contrast, oscillatory-zoned zircons from the mafic granulite with higher Th/U ratios of 0.44–0.86 recorded a magmatic age of 2648.8 ± 9.8 Ma. As the age is consistent with previously reported age of a tonalite gneiss (2645 Ma) from the same region, we infer bimodal arc magmatism during Neoarchean. The results of this study indicate that the southern Motloutse Complex underwent high-pressure granulite-facies metamorphism with post-peak decompression along a single clockwise P–T evolution during Paleoproterozoic Era (∼2.02 Ga). Our integrated petrological and geochronological studies suggest that the southern Motloutse Complex is nearly equivalent to the Beit Bridge Complex as a possible western extension of a collisional suture (the Central Zone of the Limpopo Complex) between the Archean Kaapvaal and Zimbabwe Cratons. The occurrence of ca. 2.0 Ga high-pressure granulite-facies rocks from the southern Motloutse Complex might suggest multiple collisions of discrete crustal units during Neoarchean to Paleoproterozoic.

Late Ediacaran lateral-escape tectonics as recorded by the Patos shear zone (Borborema Province, NE Brazil)

Lateral escape tectonics mediated by strike-slip fault zones are an efficient mechanism of rock deformation in the continental crust of collision zones. The Borborema shear zone system, which shows dimensions comparable to the Cenozoic extrusion of Indochina, defines a lateral escape setting of the Brasilano orogeny in Northeastern Brazil. In this paper, the timing of high-grade metamorphism and compositions of the terranes involved in the shear deformation were investigated. The Patos shear corridor deforms the Siderian to Neoarchean rocks of the Granjeiro Complex that, in turn, form the basement of Seridó-Lavras da Mangabeira metapelites. U-Pb zircon ages and Sm-Nd whole-rock isotopic compositions indicate that the 2.80 – 2.35 Ga basement sequences mainly include juvenile material, whereas zircons from synkinematic migmatites indicate that the partial melting occurred in the Late Ediacaran (ca. 565 Ma). Cooling rates provided by 40Ar/39Ar range from 12 to 17°C/Ma, indicating a differential shear zone exhumation, in agreement with a transpressive setting. The presence of allochthonous Siderian sequences dismembered along the shear zone suggests that the Seridó-Granjeiro corridor defines a major tectonic boundary connected to the collisional front defined by the convergence of São Francisco-Congo and Amazonian cratons.

Distribution of the Pan‐African Domains in East Antarctica and Adjacent Areas

AbstractThe Pan‐African event is widely distributed in East Antarctica craton. Many terranes or outcrops of the craton bear more or less signs of the event. From characteristics of the shear zones, granites, pegmatites, feature and time of high grade metamorphism and detrital zircon ages peaks of the downflowing sediments from the plateau, the Pan‐African event in the East Antarctica and adjacent areas in the Gondwana reconstruction, like SE Africa, southern India and SW Australia, wasdistributed as special zones or areas in many localities, including both the coastal regions and interior of the East Antarctica. In geochemistry, the granites are generally anorogenic, ocassionally with some gabbros or dolerite dykes, showing sign of bi‐modal feature. The water or fluid available along the shear zones were responsible for retrogression of the earlier, e.g., Grenville age, high grade outcrops to later Pan‐African amphibolite facies metamorphism. Meanwhile, the Pan‐African event has influenced most isotopic systems, including the U‐Pb, Sm‐Nd, Rb‐Sr and Ar‐Ar systems, giving younger apparent ages. Manifestation of the Pan‐African event is distributed from possibly locally granitic magmatism, to wider medium‐high grade metamorphism, and mostly widespread in resetting for some isotope systems, suggesting the prevailing thermal effect.Before Gondwana formation, local depressions in the East Antarctica could be filled with sediments, implying the initial breakup period of the Rodinia. The later Pan‐Gondwana counterrotating cogs shaped the interstitial fold belts between continent blocks and formed a set of shear zones. The mafic underplating in the Gondwana may be responsible for the widespread granites, pegmatites and more or less isotopic resetting due to strong thermal effect from the deep. That is, the Pan‐African event is a possible response of the plate movement surrounding the continent swarms in the non‐stable interior of theyet consolidated Gondwana. The Pan‐African event may be an overwhelmingly extensional and transcurrent tectonics in mechanism.

Timing of high-grade metamorphism in the Kontum Massif, Vietnam: Constraints from zircon–monazite multi-geochronology and trace elements geochemistry of zircon–monazite–garnet

Metamorphic rocks in the Kontum Massif, including the Kannak, Ngoc Linh and Kham Duc Complexes are characterized by amphibolite- to granulite-facies metamorphism and their mono- versus poly-metamorphic evolution is a matter of debate. In this study, we constrained on the timing of high-grade metamorphism in this massif by combining laser ablation inductively coupled plasma mass spectrometry U–Pb zircon and electron microprobe chemical U–Th–Pb monazite ages with geochemical relation of trace elements between zircon–monazite–garnet. The results indicated that the analyzed pelitic and mafic granulites in the western Kannak Complex experienced the high temperature metamorphism at ca. 250 Ma with subsequent decompression. However, in the western Ngoc Linh Complex two high-grade metamorphic events were recorded in one of analyzed pelitic gneisses. The first high temperature metamorphism began at ca. 440–450 Ma with the attainment of the peak condition at ca. 430 Ma, overprinted by the second one at ca. 240–250 Ma with following decompression and exhumation stage at ca. 230 Ma. The other pelitic gneiss and mafic granulite detected only younger single event. Pelitic gneisses in the Kham Duc Complex recorded medium pressure metamorphism occurred at ca. 240 Ma without any traces of early metamorphic event.

Geochronology and geochemistry of the Thirsty Creek Norite, Westland, New Zealand

ABSTRACTThirsty Creek Norite is a small, undeformed, mafic intrusion intruding gneisses of the Granite Hill Complex in the Buller Terrane. An age of 107.1 ± 4.1 Ma has been obtained using Rb-Sr geochronology. A relatively high MSWD of 4.5 is attributed to sericitization and alteration of plagioclase. This date is consistent with other ages for the timing of high-grade metamorphism in the Granite Hill Complex and may represent a metamorphic age, thus providing a minimum age for metamorphism and emplacement of the norite. However, the relatively undeformed nature of the Thirsty Creek Norite suggests it may postdate metamorphism and we speculate that the date could represent a crystallisation age which is contemporaneous with Rahu Suite granitic magmatism in the adjacent Hohonu Batholith. Several geochemical characteristics, including overlapping initial Sr and Nd isotope compositions, suggest the Thirsty Creek Norite could be a relatively mafic yet contaminated melt that is petrogenetically related to Rahu Suite magmatism. Overlaps in the isotopic compositions of the Thirsty Creek Norite and Rahu Suite granitoids therefore imply that any mafic components involved during granitic magmatism must have had relatively enriched isotopic compositions, and that evolution to more felsic compositions occurred without significant interactions with upper crustal components.

Cordierite-bearing granulites from Ihosy, southern Madagascar: Petrology, geochronology and regional correlation of suture zones in Madagascar and India

Madagascar, a major fragment of Gondwana, is mainly composed of Precambrian basement rocks formed by Mesoarchean to Neoproterozoic tectono-thermal events and recording a Pan-African metamorphic overprint. The Ranotsara Shear Zone in southern Madagascar has been correlated with shear zones in southern India and eastern Africa in the reconstruction of the Gondwana supercontinent. Here we present detailed petrology, mineral chemistry, metamorphic P–T constraints using phase equilibrium modelling and zircon U-Pb geochronological data on high-grade metamorphic rocks from Ihosy within the Ranotsara Shear Zone. Garnet-cordierite gneiss from Ihosy experienced two stages of metamorphism. The peak mineral assemblage is interpreted as garnet + sillimanite + cordierite + quartz + plagioclase + K-feldspar + magnetite + spinel + ilmenite, which is overprinted by a retrograde mineral assemblage of biotite + garnet + cordierite + quartz + plagioclase + K-feldspar + magnetite + spinel + ilmenite. Phase equilibria modelling in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) indicates peak metamorphic conditions of 850–960 °C and 6.9–7.7 kbar, and retrograde P–T conditions of <740 °C and <4.8 kbar, that define a clockwise P–T path. Near-concordant ages of detrital zircon grains in the garnet-cordierite gneiss dominantly exhibit ages between 2030 Ma and 1784 Ma, indicating dominantly Paleoproterozoic sources. The lower intercept age of 514 ± 33 Ma probably indicates the timing of high-grade metamorphism, which coincides with the assembly of the Gondwana supercontinent. The comparable rock types, zircon ages and metamorphic P–T paths between the Ranotsara Shear Zone and the Achankovil Suture Zone in southern India support an interpretation that the Ranotsara Shear Zone is a continuation of the Achankovil Suture Zone.

Mesoarchean convergent margin processes and crustal evolution: Petrologic, geochemical and zircon U–Pb and Lu–Hf data from the Mercara Suture Zone, southern India

The Mercara Shear Zone is sandwiched between the Western Dharwar Craton and the Coorg Block in the Southern Granulite Terrain of India, and is marked by steep gravity gradients interpreted to suggest the presence of underplated high-density material in the lower crust. Here we present geological, petrological and geochemical data, together with zircon U–Pb ages and Lu–Hf isotopes from a suite of metaigneous (TTG-related gneisses, charnockite, metagabbro, mafic granulite) and metasedimentary (quartz mica schist, khondalite, garnet biotite gneiss, kyanite–sillimanite bearing metapelite) rocks from this zone. Geochemical data on the magmatic suite suggests formation through subduction-related arc magmatism, whereas the metasediments represent volcano-sedimentary trench sequences. Phase equilibrium modeling of mafic granulites from the Mercara Shear Zone suggests P–T range of 10–12kbar at 700°C to 900°C. The zircon data yield weighted mean 207Pb/206Pb ages of 3229±80Ma for metagabbro, 3168±25Ma for the charnockite, and 3181±20Ma for the mafic granulite. Ages ranging from 3248±28Ma to 3506±26Ma were obtained from zircons in the kyanite/sillimanite bearing metapelite, 3335±44Ma from khondalite, 3135±14Ma from garnet biotite gneiss, 3145±17Ma to 3292±57Ma from quartz mica schist and 3153±15Ma to 3252±36 from TTG gneiss. The tightly defined ages of 3.1 to 3.2Ga from igneous zircons in the magmatic suite suggest prominent Mesoarchean convergent margin magmatism. The timing of high grade metamorphism as constrained from metamorphic overgrowths in zircons is ca. 3.0Ga which might mark the collisional event between the Western Dharwar Craton and the Coorg Block. Hf isotope features suggest magma derivation mostly from juvenile sources and the Lu–Hf model ages indicate that the crust building might have also involved partial recycling of basement rocks as old as ca. 3.8Ga. Our study defines the Mercara Shear Zone as a terrane boundary, and possible Mesoarchean suture along which the Coorg Block was accreted to the Western Dharwar Craton. The accretion of these continental fragments might have coincided with the birth of the oldest supercontinent “Ur”.

Petrology and zircon U–Pb geochronology of metagabbros from a mafic–ultramafic suite at Aniyapuram: Neoarchean to Early Paleoproterozoic convergent margin magmatism and Middle Neoproterozoic high-grade metamorphism in southern India

Several mafic–ultramafic complexes occur within the Palghat-Cauvery Suture Zone (PCSZ) in southern India. The PCSZ is regarded in recent models as the zone along which crustal blocks were amalgamated during the Late Neoproterozoic–Cambrian (550–530Ma) Gondwana assembly. Here we report petrologic and zircon U–Pb geochronologic data from gabbros associated with the Aniyapuram mafic–ultramafic suite in the central domain of the PCSZ. Geothermobarometry and pseudosection approach in the system NCFMASHTO for the metagabbro (Grt+Cpx+Opx+Hbl+Pl+Qtz+Ilm+Rt) yield peak P–T condition of 9.8–10.6kbar and 730–790°C, which was followed by decompression to 6.5–8.0kbar and ca. 750°C as inferred from the formation of Opx+Pl symplectite around garnet, probably along a clockwise P–T path. Zircon U–Pb data analyzed by LA-ICP-MS plot along a well-defined discordia with upper and lower intercepts in the concordia at 2436±22Ma and 731±11Ma respectively, suggesting Neoarchean–Early Paleoproterozoic magmatic emplacement of the protolith and progressive Pb loss related to the Middle Neoproterozoic (Cryogenian) thermal event (or high-grade metamorphism). These results closely compare with the available Neoarchean magmatic ages of mafic–ultramafic complexes (e.g., Sittampundi, Devanur, Agali Hills, and Kanja Malai) and Middle Neoproterozoic magmatic event (e.g., Manamedu and Kadavur) in the PCSZ and adjacent granulite blocks. The 650Ma concordia ages obtained from unzoned zircons might indicate the timing of high-grade metamorphism or post-peak hydration event. The P–T conditions obtained from Aniyapuram are significantly lower than the high-pressure and ultrahigh-temperature conditions of the 550–530Ma final collisional event (P>14 kbar and T>950°C). The Middle Neoproterozoic (ca. 730Ma or 650Ma) high-grade metamorphism in Aniyapuram reported for the first time from the PCSZ is possibly associated with magmatism in arc tectonic setting.

Neoproterozoic granulites from the northeastern margin of the Tarim Craton: Petrology, zircon U–Pb ages and implications for the Rodinia assembly

We report the discovery of a suite of Neoproterozoic granulite facies rocks from the vicinity of the Boston Lake area at the northeastern margin of the Tarim Craton. Mineral parageneses show that these rocks witnessed peak granulite facies metamorphism followed by retrograde amphibolite facies overprinting. Pressure–temperature computations of the Grt–Opx granulites yield ca. 810°C and 1.0GPa, falling in the lower domain of high-pressure granulite-facies P–T field. Most of the zircons in our samples display core–rim textures with metamorphic overgrowths on detrital cores. LA-ICP-MS U–Pb dating on the zircon cores yields three major age populations at ca. 0.9–1.0Ga, 1.8–2.0Ga and 2.4–2.6Ga, suggesting the composite sources for the protolith sedimentary rocks. The metamorphic zircons from different samples record a narrow age range between 790 and 820Ma, marking the timing of high grade metamorphism. This is the first report of Neoproterozoic high-grade metamorphic event from the northeastern margin of the Tarim Craton. Our results could suggest either a major collisional event associated with the assembly of the Tarim Craton within the supercontinent Rodinia, or an Andean-type orogeny at the periphery of the Rodinia supercontinent.

Temporal constraints on the timing of high-grade metamorphism in the northern Gawler Craton: implications for assembly of the Australian Proterozoic

LA-ICPMS U–Pb data from metamorphic monazite in upper amphibolite and granulite-grade metasedimentary rocks indicate that the Nawa Domain of the northern Gawler Craton in southern Australia underwent multiple high-grade metamorphic events in the Late Paleoproterozoic and Early Mesoproterozoic. Five of the six samples investigated here record metamorphic monazite growth during the period 1730–1690 Ma, coincident with the Kimban Orogeny, which shaped the crustal architecture of the southeastern Gawler Craton. Combined with existing detrital zircon U–Pb data, the metamorphic monazite ages constrain deposition of the northern Gawler metasedimentary protoliths to the interval ca 1750–1720 Ma. The new age data highlight the craton-wide nature of the 1730–1690 Ma Kimban Orogeny in the Gawler Craton. In the Mabel Creek Ridge region of the Nawa Domain, rocks metamorphosed during the Kimban Orogeny were reworked during the Kararan Orogeny (1570–1555 Ma). The obtained Kararan Orogeny monazite ages are within uncertainty of ca 1590–1575 Ma zircon U–Pb metamorphic ages from the Mt Woods Domain in the central-eastern Gawler Craton, which indicate that high-grade metamorphism and associated deformation were coeval with the craton-scale Hiltaba magmatic event. The timing of this deformation, and the implied compressional vector, is similar to the latter stages of the Olarian Orogeny in the adjacent Curnamona Province and appears to be part of a westward migration in the timing of deformation and metamorphism in the southern Australian Proterozoic over the interval 1600–1545 Ma. This pattern of westward-shifting tectonism is defined by the Olarian Orogeny (1600–1585 Ma, Curnamona Province), Mt Woods deformation (1590–1575 Ma), Mabel Creek Ridge deformation (1570–1555 Ma, Kararan Orogeny) and Fowler Domain deformation (1555–1545 Ma, Kararan Orogeny). This westward migration of deformation suggests the existence of a large evolving tectonic system that encompassed the emplacement of the voluminous Hiltaba Suite and associated volcanic and mineral systems.

U–Th–Pb fractionation in Archaean lower continental crust: Implications for terrestrial Pb isotope systematics

Pb isotopic compositions are reported for leached feldspars and whole rock samples of felsic to ultramafic, amphibolite to granulite facies xenoliths from the Bearpaw Mountains in the northern Wyoming Craton, Montana, USA. Two-point dates obtained for leached feldspar–whole rock pairs provide a record of Pb isotopic homogenisation at 1.6–2.1 Ga, during an inferred Palaeoproterozoic thermo-tectonic event. Model ages for leached feldspars, assuming single-stage melt extraction from depleted mantle, indicate formation of the protoliths from 2.8 to 4.0 Ga. The inferred timing of the Palaeoproterozoic high-grade metamorphism and extended Archaean crust formation is in agreement with available geochronological data for the Wyoming Craton. Following Palaeoproterozoic isotope resetting, the second stage of Pb isotope evolution suggests prolonged residence in a low U/Pb environment. Isotope resetting was apparently accompanied by a significant decrease in 238U/204Pb (μ-values), reflecting substantial loss of the highly incompatible and mobile element U and, by inference, other heat-producing elements. Geothermal considerations suggest that unstratified continental crust, as approximated by a “reconstituted” average xenolith composition from the Wyoming Craton, would have been thermally unstable at lower crustal levels at the time of formation and metamorphism, if abundances of heat-producing elements had been unaffected by the U–Th–Pb fractionation event. In contrast, differentiated continental crust comprising a chemically depleted lower layer and an enriched upper layer would have reached thermal stability at the time of high-grade metamorphism. On this basis, the Wyoming Craton xenoliths data are proposed as an approximation to the lower continental crust. In 207Pb/204Pb vs. 206Pb/204Pb space the xenoliths plot to the left of the meteorite isochron and above the depleted mantle evolution curve, in contrast to most previous data from lower crustal xenolith suites. Simple mass balance, using an average of the studied xenoliths from the Wyoming Craton as approximation to lower continental crust and published compositions for upper continental crust, yields a Pb isotopic composition of bulk continental crust that plots close to the meteorite isochron. Hence, it is proposed that Archaean lower crust, as exemplified by the Wyoming Craton xenoliths, displays the essential geochemical and isotopic requisites to be a major reservoir to balance the Pb isotope composition of bulk silicate earth.

Isotopic constraints on the thermal history of the Wind River Range, Wyoming: implications for Archean metamorphism

Precise U–Pb monazite and 40Ar/39Ar hornblende ages have been obtained from three locations in the high-grade Archean core of the Wind River Range, Wyoming. Monazites from metapelites in the Paradise Basin, Medina Mountain, and Crescent Lake have U–Pb ages of 2718 ± 1, 2633 ± 5, and 2657 ± 2 Ma, respectively. Hornblendes from amphibolites and granulites from the same locations yield plateau 40Ar/39Ar isotope ages of 2652 ± 11, 2572 ± 9, and 2527 ± 8 Ma, respectively, and are interpreted as cooling ages from the last thermal event. The three localities experienced similar peak pressure–temperature conditions. The timing of high-grade metamorphism in the Paradise Basin is older than the emplacement of large subjacent batholiths at 2.63–2.67 Ga. Calculated cooling rates based on monazite–hornblende pairs of 3.4 ± 1.0 °C/Ma for Paradise Basin, 3.8 ± 1.2 °C/Ma for Medina Mountain, and 1.7 ± 0.4 °C/Ma for Crescent Lake cannot be used to rule out reheating during subsequent pluton emplacement. The markedly slower cooling rate inferred for Crescent Lake may indicate early differential uplift or may demark another regional metamorphic event. The difference in 40Ar/39Ar ages between hornblende (2652 ± 11 Ma) and biotite (2637 ± 11 Ma) suggests a more rapid cooling rate, 11 °C/Ma, for Paradise Basin between 2.65 and 2.63 Ga, which may be related to the time of large-scale batholith emplacement elsewhere in the terrane. Combining new data with other ages in the Wind River Range reveals an extended metamorphic history, punctuated by thermal events over a time interval of at least 700 Ma.

Timing of high-grade metamorphism in central Turkey and the assembly of Anatolia

The timing of metamorphism of the crystalline massifs in western and central Turkey (Menderes, Kirșehir, Nigde, Akdag) is important information for understanding the assembly of Anatolia and the thermal–tectonic evolution of the Turkish segment of the Alpine–Himalayan belt. The high-grade basement rocks of the massifs are polymetamorphic, and the complexity of interpreting ages obtained from zircon and other minerals has led to a longstanding debate about the significance and regional extent of Neoproterozoic v. younger (Mesozoic, Cenozoic) metamorphism. New U–Pb SHRIMP (sensitive high-resolution ion microprobe) analyses of monazite from a garnet–sillimanite gneiss in the Kirșehir Massif, central Anatolia, document Late Cretaceous (84.1 ± 0.8 Ma) high-temperature metamorphism. This Late Cretaceous age for central Anatolian metamorphism is at least 50 Ma older than Alpine metamorphism in the Menderes Massif in western Turkey. Models that propose a continuous Anatolide–Tauride tectonic zone, with central Anatolia as a promontory, cannot explain such a large time difference within a small region. Unless the Menderes ages have been reset by Aegean thermal–tectonic events, Anatolia probably formed from a collage of continental fragments rather than by simple collision of an irregular Tethyan platform with Eurasia.

Zoned (Cretaceous and Cenozoic) garnet and the timing of high grade metamorphism, Southern Alps, New Zealand

AbstractNew (garnet Sm–Nd and Lu–Hf) and existing (Rb–Sr, 40Ar/39Ar, U–Pb and Sm–Nd) ages and data on deformational fabrics and mineral compositions show for the first time that the garnet growth and ductile deformation in the Alpine Schist belt and Southern Alps orogen, New Zealand are diachronous and partly Cenozoic in age. The dominant metamorphic isograds in the Alpine Schist formed during crustal thickening at a previously unsuspected time, at c. 86 Ma, immediately prior to the opening of the Tasman Sea at c. 84–82 Ma. Obvious changes in the textures and compositional zoning patterns of garnet are not always reliable indicators of polymetamorphism, and fabric elements can be highly diachronous.A detailed timing history for the growth of a single garnet is recorded by a Sm–Nd garnet–whole rock age of 97.8 ± 8.1 Ma for the inmost garnet core (zone 1), Lu–Hf ages of 86.2 ± 0.2 Ma and 86.3 ± 0.2 Ma for overgrowth zones 2 and 3, a step‐leach Sm–Nd age of 12 ± 37 Ma for zone 4, and growth of the garnet rim (zone 5) over the Alpine Fault mylonite foliation during the modern phase of oblique collision that began at c. 5–6 Ma.Plate convergence along the New Zealand portion of the Gondwana margin continued after c. 105 Ma, almost certainly culminating in the oblique collision of a large oceanic plateau (Hikurangi Plateau). The metamorphism of the Alpine Schist at c. 86 Ma is evidence of that hit. The mid‐ to late‐Cretaceous extension that is widespread elsewhere in the New Zealand region is attributed to upper plate extension and slab roll‐back. The effects of the collision with the Hikurangi Plateau may have contributed to the changing plate motions in the region leading up to the opening of the Tasman Sea at c. 82 Ma.

Monazite dating of granitic gneisses and leucogranites from the Kerala Khondalite Belt, southern India: implications for Late Proterozoic crustal evolution in East Gondwana

Two stages of granitic magmatism occurred during the Pan-African evolution of the Kerala Khondalite Belt (KKB) in southern India. Granitic gneisses were derived from porphyritic granites, which intruded prior to the main stage of deformation and peak-metamorphism. Subsequently, leucogranites and leucotonalites formed during fluid-absent melting and intruded the gneiss sequences. Monazites from granitic gneisses, leucogranites and a leucotonalite were investigated by conventional U-Pb and electron microprobe dating in order to distinguish the different stages of magma emplacement. U-Pb monazite dating yielded a wide range of ages between 590–520 Ma which are interpreted to date high-grade metamorphism rather than magma emplacement. The results of this study indicate that the KKB experienced protracted heating (>50 Ma) at temperatures above 750–800 °C during the Pan-African orogeny. The tectonometamorphic evolution of the study area is comparable to southern Madagascar which underwent a similar sequence of events earlier than the KKB. The results of this study further substantiate previous assertions that the timing of high-grade metamorphism in East Gondwana shifted from west to east during the Late Proterozoic.

Felsic metavolcanic rocks related to the Broken Hill Pb-Zn-Ag orebody, Australia; geology, depositional age, and timing of high-grade metamorphism

Two key issues related to the origin of the massive Broken Hill Pb-Zn-Ag orebody are the genesis of its host rocks and their depositional age. These granulite facies layered gneisses have been ascribed alternately to volcanic and nonvolcanic regimes. Determination of the depositional age of the sequence has never been seriously addressed.North of Broken Hill, amphibolite facies precursors to the granulite facies gneisses show compelling evidence of a volcanic origin in their texture, chemistry, and zircon morphology. These felsic metavolcanic rocks are in the upper part of the Broken Hill Group, stratigraphically equivalent to the Broken Hill orebody. The present work traces textural, zircon morphological and isotopic changes across the amphibolite to granulite facies transition, in order to determine the primary depositional age of this supracrustal sequence and the timing of highgrade regional metamorphism.Ion microprobe (SHRIMP) U-Pb data on zircons from the lower grade (amphibolite facies) felsic metavolcanic rocks define a major magmatic population at 1690 + or - 5 Ma. This is considered to be the age of zircon crystallization and eruption of felsic volcanics in this part of the Broken Hill Group. These SHRIMP data clarify inheritance and discordance problems that had plagued conventional, multigrain U-Pb analyses.Zircon results from granulite facies stratigraphic equivalents of these rocks, near Southern Cross and at Broken Hill, have patterns similar to those of the amphibolite facies rocks, including igneous relics that provide the same primary age. Additionally, zircon in the highest grade granulite facies rocks ( approximately 800 degrees C) forms metamorphic overgrowths surrounding the magmatic relics. These younger zircon selvedges have a pooled age of 1600 + or - 8 Ma, which determines the timing of this granulite facies event. Consistent U-Pb monazite results yield this same age (1599 + or - 9 Ma), suggesting that the monazite blocking temperature is close to that of zircon or that cooling to <725 degrees C was achieved relatively rapidly. This is some 60 m.y. younger than the previously accepted age for high-grade metamorphism at Broken Hill. Irrespective of metamorphic grade in the gneisses, all biotite Rb-Sr ages have been reset to approximately 480 Ma, a reflection of early Paleozoic (Delamerian) orogenesis.Inherited zircon grains, interpreted as xenocrysts and in some cases mantied by 1690 Ma igneous rims, have older Proterozoic to late Archcan ages (1780-2700 Ma). The presence of xenocrystic zircon components points to an ensialic tectonic setting for the Broken Hill block and reflects accidental incorporation in the felsic volcanic protoliths of lithic fragments and/or complex igneous sources, such as found in crust exposed in the Gawler craton to the west.The 1690 + or - 5 Ma depositional age determined here provides a firm chronologic tie between the early Proterozoic sequences enclosing major stratiform Pb-Zn-Ag orebodies at Broken Hill, Mount Isa (1670 + or - 19 Ma), and McArthur River (1690 + or - 27 Ma) in northern Australia. In light of the uncertainties over timing and mode of origin of these deposits, the similar ages of their host sequences have important implications for early Proterozoic tectonics and metallogeny.

A U [sbnd]Th [sbnd]Pb lead isotope study of rocks and ores from Broken Hill, Australia

A lead “double-spike” calibrated against an NBS Standard was used to obtain the isotopic abundances of totalrock samples of high-grade gneiss from the Mine Sequence, Broken Hill. New lead isotope data for a number of Thackaringa Type ore leads are also reported as are the lead, uranium and thorium concentrations of the rock samples. An age of 1690 ± 35 my is calculated from the apparent linear relationship of the rock leads on a 207Pb/ 204Pb− 206Pb/ 204Pb diagram. This agrees with previously reported Rb Sr results and is similarly interpreted as the time of the regional high-grade metamorphism. Recent mobility of uranium and thorium may account for the considerable scatter of the rock data when plotted on conventional U Th Pb isochron diagrams. The slope of the least squares line through the galena data on a 207Pb/ 204Pb− 206Pb/ 204Pb plot is substantially higher than values previously reported. It is therefore suggested that Thackaringa mineralization occurred approximately 800 my ago, prior to the retrograde metamorphism dated at 500 my. The common lead component in both Thackaringa ores and high-grade gneisses may be isotopically identical to lead from the main lode orebody (that is, Broken Hill Type lead). Hence, formation of this orebody with lead derived from local source rocks at the time of the high-grade metamorphism may be indicated. Alternatively, as suggested by Stanton, metamorphism of ore and host rocks soon after syngenetic deposition would be likely to give the observed isotopic results.