The timing of ultrahigh-temperature metamorphism in Southern India: U–Th–Pb electron microprobe ages from zircon and monazite in sapphirine-bearing granulites

We report here U–Pb electron microprobe ages from zircon and monazite associated with corundum- and sapphirine-bearing granulite facies rocks of Lachmanapatti, Sengal, Sakkarakkottai and Mettanganam in the Palghat–Cauvery shear zone system and Ganguvarpatti in the northern Madurai Block of southern India. Mineral assemblages and petrologic characteristics of granulite facies assemblages in all these localities indicate extreme crustal metamorphism under ultrahigh-temperature (UHT) conditions. Zircon cores from Lachmanapatti range from 3200 to 2300 Ma with a peak at 2420 Ma, while those from Mettanganam show 2300 Ma peak. Younger zircons with peak ages of 2100 and 830 Ma are displayed by the UHT granulites of Sengal and Ganguvarpatti, although detrital grains with 2000 Ma ages are also present. The Late Archaean-aged cores are mantled by variable rims of Palaeo- to Mesoproterozoic ages in most cases. Zircon cores from Ganguvarpatti range from 2279 to 749 Ma and are interpreted to reflect multiple age sources. The oldest cores are surrounded by Palaeoproterozoic and Mesoproterozoic rims, and finally mantled by Neoproterozoic overgrowths. In contrast, monazites from these localities define peak ages of between 550 and 520 Ma, with an exception of a peak at 590 Ma for the Lachmanapatti rocks. The outermost rims of monazite grains show spot ages in the range of 510–450 Ma.While the zircon populations in these rocks suggest multiple sources of Archaean and Palaeoproterozoic age, the monazite data are interpreted to date the timing of ultrahigh-temperature metamorphism in southern India as latest Neoproterozoic to Cambrian in both the Palghat–Cauvery shear zone system and the northern Madurai Block. The data illustrate the extent of Neoproterozoic/Cambrian metamorphism as India joined the Gondwana amalgam at the dawn of the Cambrian.     

Sm–Nd ages of mafic rocks from the Coastal Cordillera at 24°S, northern Chile

We investigated the Sm–Nd systematics of mafic granulite and undeformed layered gabbro, which form a midcrustal section of the Jurassic magmatic belt in the North Chilean Coast Range, south of Antofagasta. Mineral isochrons indicate ages between ca. 171 and 150?Ma for the granulite and an age of ca. 161 Ma for the gabbro. These ages are interpreted as closure time of the Sm–Nd system in the area. The age of metamorphism is Late-Jurassic. The minimum intrusion age of the protolith of the granulite is likely Early Jurassic (ca. 200?Ma), but an exact intrusion age could not be derived from the data. The intrusion age of the gabbro is ca. 185 Ma. Granulite generated from mantle-derived gabbroic magmas has ?_Nd200 between 6.28 and 7.05 and the gabbro that intruded the granulite has ?_Nd185 between 5.89 and 6.09.     

Decoding a protracted zircon geochronological record in ultrahigh temperature granulite,and persistence of partial melting in the crust,Rogaland, Norway

This contribution evaluates the relation between protracted zircon geochronological signal and protracted crustal melting in the course of polyphase high to ultrahigh temperature (UHT; T?>?900 °C) granulite facies metamorphism. New U–Pb, oxygen isotope, trace element, ion imaging and cathodoluminescence (CL) imaging data in zircon are reported from five samples from Rogaland, South Norway. The data reveal that the spread of apparent age captured by zircon, between 1040 and 930 Ma, results both from open-system growth and closed-system post-crystallization disturbance. Post-crystallization disturbance is evidenced by inverse age zoning induced by solid-state recrystallization of metamict cores that received an alpha dose above 35 × 10¹⁷ α  g^?1. Zircon neocrystallization is documented by CL-dark domains displaying O isotope open-system behaviour. In UHT samples, O isotopic ratios are homogenous (δ¹⁸O = 8.91?±?0.08‰), pointing to high-temperature diffusion. Scanning ion imaging of these CL-dark domains did not reveal unsupported radiogenic Pb. The continuous geochronological signal retrieved from the CL-dark zircon in UHT samples is similar to that of monazite for the two recognized metamorphic phases (M1: 1040–990 Ma; M2: 940–930 Ma). A specific zircon-forming event is identified in the orthopyroxene and UHT zone with a probability peak at ca. 975 Ma, lasting until ca. 955 Ma. Coupling U–Pb geochronology and Ti-in-zircon thermometry provides firm evidence of protracted melting lasting up to 110 My (1040–930 Ma) in the UHT zone, 85 My (ca. 1040–955 Ma) in the orthopyroxene zone and some 40 My (ca. 1040–1000 Ma) in the regional basement. These results demonstrate the persistence of melt over long timescales in the crust, punctuated by two UHT incursions.     

柴西缘花土沟超高温变质地体中的镁铝麻粒岩岩石成因及其与熔体行为的关系

镁铝麻粒岩泛指一类全岩化学成分富镁、铝的麻粒岩相变质岩,是研究超高温变质作用的峰期变质条件和变质演化历史的重要对象,但目前对它的原岩属性和岩石成因的认识仍十分有限。本文以柴达木地块西缘的花土沟超高温变质地体为例,在野外调查基础上,对镁铝麻粒岩和泥质片麻岩进行了岩相学和全岩地球化学分析,发现镁铝麻粒岩与含浅色体的泥质片麻岩的SiO₂、TiO₂、P₂O₅ 含量相似,^TFe₂O₃、Al₂O₃、MnO、CaO、Na₂O含量虽有差异但变化范围存在交集。此外,两类岩石具有相似的微量和稀土元素配分曲线,结合两者的矿物组合也存在相似性,提出花土沟镁铝麻粒岩的原岩可能是与泥质片麻岩类似的泥质沉积岩。从低角闪岩相变泥质岩到含浅色体的泥质片麻岩,再到镁铝麻粒岩,其全岩化学成分向着贫铝、钙、钾、钠,富铁、镁的趋势变化。其中,高X_Mg值（0.51~0.69）是镁铝麻粒岩与其他泥质片麻岩（X_Mg=0.34~0.43）的最大差别。通过对变泥质岩的相平衡模拟和理论计算,发现部分熔融和熔体丢失能解释大部分的变化趋势,但基本不影响全岩X_Mg值;只有在进变质升温过程中丢失含石榴子石的熔体才能造成变泥质岩的镁铝麻粒岩化。此外,富石榴子石的泥质残留体相比附近的含浅色体泥质片麻岩,贫硅、钠、钾,富集铝、铁、镁、锰、钙,重稀土元素含量显著高于后者,上述地球化学特征对应石榴子石熔体的加入而后长英质熔体的丢失,支持野外观察到的熔体携带石榴子石迁移的现象。最后,对镁铝麻粒岩只呈透镜体产出的原因做出了推测,即熔体很难带着石榴子石完成长距离迁移,只有被长英质正片麻岩包围的泥质沉积物,其进变质加热阶段形成的熔体才能携带石榴子石完全迁出原岩,促成变泥质岩透镜体的镁铝麻粒岩化,目前仍需更多的相关研究来验证这一推测。在世界其他高温-超高温变质岩区,石榴子石熔体的迁出和泥质岩的镁铝麻粒岩化可能也不同程度有所保留和记录。     

北大别黄土岭长英质麻粒岩的原岩、变质作用及源区热事件年龄的锆石LA-ICPMS U-Pb测年约束

本研究应用激光剥蚀技术测定了北大别黄土岭高温-高压长英质麻粒岩锆石3个结构域的U-Pb年龄.变质锆石成因的碎屑锆石域的207Pb/206Pb年龄范围为(2493±54) Ma~(2500±180) Ma, 岩浆成因的碎屑锆石域的207Pb/206Pb年龄范围为2628~2690Ma, 其最大的206Pb/238U年龄为(2790±150) Ma, 变质增生或变质重结晶锆石域的不一致线上交点年龄为(2044.7±29.3) Ma.长英质麻粒岩的矿物组合成分、主量元素地球化学, 尤其是锆石副矿物内部结构特征显示其原岩为沉积岩.这表明, 麻粒岩原岩物质来自具有复杂热历史的蚀源区, 该蚀源区曾发生过~2.8Ga的岩浆作用和~2.5Ga变质作用, 因此其原岩的沉积年龄不应早于2.5Ga.高温-高压麻粒岩相变质作用的精确年龄为(2.04±0.03) Ga, 表明黄土岭麻粒岩是一个晚古元古代超高温变质岩之残块.      

Cryogenian magmatism and crustal reworking in the Southern Granulite Terrane,India

Understanding Neoproterozoic crustal evolution is fundamental to reconstructing the Gondwana supercontinent, which was assembled at this time. Here we report evidence of Cryogenian crustal reworking in the Madurai Block of the Southern Granulite Terrane of India. The study focuses on a garnet-bearing granite–charnockite suite, where the granite shows in situ dehydration into patches and veins of incipient charnockite along the contact with charnockite. The granite also carries dismembered layers of Mg–Al-rich granulite. Micro-textural evidence for dehydration of granite in the presence of CO₂-rich fluids includes the formation of orthopyroxene by the breakdown of biotite, neoblastic zircon growth in the dehydration zone, at around 870°C and 8 kbar. The zircon U–Pb ages suggest formation of the granite, charnockite, and incipient charnockite at 836 ± 73, 831 ± 31, and 772 ± 49 Ma, respectively. Negative zircon εHf (t) (?5 to ?20) values suggest that these rocks were derived from a reworked Palaeoproterozoic crustal source. Zircon grains in the Mg–Al-rich granulite record a spectrum of ages from ca. 2300 to ca. 500 Ma, suggesting multiple provenances ranging from Palaeoproterozoic to mid-Neoproterozoic, with neoblastic zircon growth during high-temperature metamorphism in the Cambrian. We propose that the garnet-bearing granite and charnockite reflect the crustal reworking of aluminous crustal material indicated by the presence of biotite + quartz + aluminosilicate inclusions in the garnet within the granite. This crustal source can be the Mg–Al-rich layers carried by the granite itself, which later experienced high-temperature regional metamorphism at ca. 550 Ma. Our model also envisages that the CO₂ which dehydrated the garnet-bearing granite generating incipient charnockite was sourced from the proximal massive charnockite through advection. These Cryogenian crustal reworking events are related to prolonged tectonic activities prior to the final assembly of the Gondwana supercontinent.     

LA-ICP-MS U-Pb Zircon Dating for Felsic Granulite, Huangtuling Area, North Dabieshan: Constraints on Timing of Its Protolith and Granulite-Facies Metamorphism, and Thermal Events in Its Provenance

Information about the protolith of the Huangtuling granulite in North Dabieshan has been unavailable. The complex evolution history of the rock and its host basement must be further discussed. LA-ICP-MS U-Pb dating was conducted on three textural domains in zircon from a high-temperature, high-pressure felsic granulite in the Huangtuling area, North Dabieshan, Central China. The metamorphic growth-derived detrital zircon domain yields a 207Pb/206Pb age in the range of (2 493±54) -(2 500±180) Ma. The magmatic genesis-derived detrital zircon domain gives a 207Pb/206Pb age ranging from 2 628 Ma to 2 690 Ma, with an oldest 206Pb/238U age of (2 790±150) Ma. The metamorphic overgrowth or metamorphic recrystallization zircon domain yields a dicsordia with an upper intercept age of (2 044.7±29.3) Ma. Compositions of the mineral assemblage, major element geochemistry, and especially the complex interior texture of the zircon suggest that the prololith of the felsic granulite is of sedimentary origin. Results show that the protolith material of the granulite came from a provenance with a complex thermal history, i.e. ～2.8 Ga magmatism and ～2.5 Ga metamorphism, and was deposited in a basin not earlier than 2.5 Ga. The high-temperature and high-pressure granulite-facies metamorphic age was precisely constrained at (2.04±0.03) Ga, which indicates the granulite in Huangtuling area should be a relict of a Paleoproterozoic UHT (ultrahigh temperature) metamorphosed slab.     

Behaviour of radiogenic Pb in zircon during ultrahigh-temperature metamorphism: an ion imaging and ion tomography case study from the Kerala Khondalite Belt,southern India

Zircon crystals from a locally charnockitized Paleoproterozoic high-K metagranite from the Kerala Khondalite Belt (KKB) of southern India have been investigated by high-spatial resolution secondary ion mass spectrometry analysis of U–Th–Pb and rare earth elements (REE), together with scanning ion imaging and scanning ion tomography (depth-profiled ion imaging). The spot analyses constrain the magmatic crystallization age of the metagranite to ca. 1,850 Ma, with ultrahigh-temperature (UHT) metamorphism occurring at ca. 570 Ma and superimposed charnockite formation at ca. 520–510 Ma, while the ion imaging reveals a patchy distribution of radiogenic Pb throughout the zircon cores. Middle- to heavy-REE depletion in ca. 570 Ma zircon rims suggests that these grew in equilibrium with garnet and therefore date the UHT metamorphism in the KKB. The maximum apparent ²⁰⁷Pb/²⁰⁶Pb age obtained from the unsupported radiogenic Pb concentrations is also consistent with formation of the Pb patches during this event. The superimposed charnockitization event appears to have caused additional Pb-loss in the cores and recrystallization of the rims. The results of depth-profiling of the scanning ion tomography image stack show that the Pb-rich domains range in size from <5 nm to several 10 nm (diameter if assumed to be spherical). The occurrence of such patchy Pb has previously been documented only from UHT metamorphic zircon, where it likely results from annealing of radiation-damaged zircon. The formation of a discrete, heterogeneously distributed and subsequently immobile Pb phase effectively arrests the normal Pb-loss process seen at lower grades of metamorphism.     

Contrasting ages between isotopic chronometers in granulites: monazite dating and metamorphism in the Higo Complex,Japan

The Higo Complex of west-central Kyushu, western Japan is a 25 km long body of metasedimentary and metabasic lithologies that increase in metamorphic grade from schist in the north to migmatitic granulite in the south, where granitoids are emplaced along the southern margin. The timing of granulite metamorphism has been extensively investigated and debated. Previously published Sm–Nd mineral isochrons for garnet-bearing metapelite yielded ca.220–280 Ma ages, suggesting high-grade equilibration older than the lower grade schist to the north, which yielded ca.180 Ma K–Ar muscovite ages. Ion and electron microprobe analyses on zircon have yielded detrital grains with rim ages of ca.250 Ma and ca.110 Ma. Electron microprobe ages from monazite and xenotime are consistently 110–130 Ma. Two models have been proposed: 1) high-grade metamorphism and tectonism at ca.115 Ma, with older ages attributed to inheritance; and 2) high-grade metamorphism at ca.250 Ma, with resetting of isotopic systems by contact metamorphism at ca.105 Ma during the intrusion of granodiorite. These models are evaluated through petrographic investigation and electron microprobe Th–U–total Pb dating of monazite in metapelitic migmatites and associated lithologies. In-situ investigation of monazite reveals growth and dissolution features associated with prograde and retrograde stages of progressive metamorphism and deformation. Monazite Th–U–Pb isochrons from metapelite, diatexite and late-deformational felsic dykes consistently yield ca.110–120 Ma ages. Earlier and later stages of monazite growth cannot be temporally resolved. The preservation of petrogenetic relationships, coupled with the low diffusion rate of Pb at < 900 °C in monazite, is strong evidence for timing high-temperature metamorphism and deformation at ca.115 Ma. Older ages from a variety of chronometers are attributed to isotopic disequilibrium between mineral phases and the preservation of inherited and detrital age components. Tentative support is given to tectonic models that correlate the Higo terrane with exotic terranes between the Inner and Outer tectonic Zones of southwest Japan, possibly derived from the active continental margin of the South China Block. These terranes were dismembered and translated northeastwards by transcurrent shearing and faulting from the beginning to the end of the Cretaceous Period.     

Detrital zircon geochronology of quartzites from the southern Madurai Block,India: Implications for Gondwana reconstruction

Detrital zircons are important proxies for crustal provenance and have been widely used in tracing source characteristics and continental reconstructions. Southern Peninsular India constituted the central segment of the late Neoproterozoic supercontinent Gondwana and is composed of crustal blocks ranging in age from Mesoarchean to late Neoproterozoic–Cambrian. Here we investigate detrital zircon grains from a suite of quartzites accreted along the southern part of the Madurai Block. Our LA-ICPMS U-Pb dating reveals multiple populations of magmatic zircons, among which the oldest group ranges in age from Mesoarchean to Paleoproterozoic (ca. 2980–1670 Ma, with peaks at 2900–2800 Ma, 2700–2600 Ma, 2500–2300 Ma, 2100–2000 Ma). Zircons in two samples show magmatic zircons with dominantly Neoproterozoic (950–550 Ma) ages. The metamorphic zircons from the quartzites define ages in the range of 580–500 Ma, correlating with the timing of metamorphism reported from the adjacent Trivandrum Block as well as from other adjacent crustal fragments within the Gondwana assembly. The zircon trace element data are mostly characterized by LREE depletion and HREE enrichment, positive Ce, Sm anomalies and negative Eu, Pr, Nd anomalies. The Mesoarchean to Neoproterozoic age range and the contrasting petrogenetic features as indicated from zircon chemistry suggest that the detritus were sourced from multiple provenances involving a range of lithologies of varying ages. Since the exposed basement of the southern Madurai Block is largely composed of Neoproterozoic orthogneisses, the data presented in our study indicate derivation of the detritus from distal source regions implying an open ocean environment. Samples carrying exclusive Neoproterozoic detrital zircon population in the absence of older zircons suggest proximal sources in the southern Madurai Block. Our results suggest that a branch of the Mozambique ocean might have separated the southern Madurai Block to the north and the Nagercoil Block to the south, with the metasediments of the khondalite belt in Trivandrum Block marking the zone of ocean closure, part of which were accreted onto the southern Madurai Block during the collisional amalgamation of the Gondwana supercontinent in latest Neoproterozoic–Cambrian.     

Petrology and isotopic evolution of granulites from central Madurai Block (southern India): reference to Ediacaran crustal evolution

The Madurai Block, constituting part of the southern granulite terrain in southern India, has contributed significantly towards understanding the UHT (ultrahigh-temperature) granulites that serve as a window into the mid-lower continental crust. The dominant rock types are charnockites, sapphirine-bearing granulites, garnet cordierite gneisses, and quartzites. Significant textural relations reveal multiphase reactions responsible for the formation of diverse mineral parageneses during prolonged metamorphic history of the area. Prograde reaction is evident from the textural relationship where biotite/sillimanite relics are seen as inclusion in garnet/orthopyroxene, suggesting dehydration reactions. The symplectitic assemblages that formed during isothermal decompression involve a series of cordierite-forming reactions, followed by retrogression and cooling. Variety of mineral assemblages present in the rocks of this area offer a wide spectrum of P–T sensors that provide details on the physical conditions of metamorphism. For the rigorous interpretation of the P–T path in the Perumalmalai area, quantitative phase diagrams (P–T pseudosections) have been constructed and contoured for the compositional as well as modal isopleths of involved mineral phases. The rocks of Perumalmalai area document a clockwise decompression P–T trajectory, consistent with crustal thickening followed by extensional collapse. SHRIMP U–Pb ages from zircon associated with sapphirine-bearing granulite facies rocks of Perumalmalai area suggest a widespread Ediacaran tectonothermal event. The occurrence of Ediacaran UHT metamorphism followed by isothermal decompression in the Madurai Block is consistent with the timing and physical conditions associated with the formation of East African Orogen during the amalgamation of Gondwana.     

东南极茹尔群岛超高温麻粒岩的研究进展

茹尔群岛(又称赖于尔群岛)位于东南极普里兹构造带的东部边缘,是一个由太古宙和中元古代岩石组成的复合高级变质地体。中元古代岩石是含有富Fe-Al的含石榴子石-矽线石的费拉副片麻岩组合,经历了格林维尔和泛非两期变质作用。太古宙正片麻岩是含有富Mg-Al的含假蓝宝石的超高温泥质麻粒岩组合(梅瑟副片麻岩组合),主要由经历超高温变质作用的含假蓝宝石的泥质麻粒岩、富Mg的石榴子石-矽线石泥质片麻岩、斜方辉石-矽线石石英岩、含石榴子石镁铁质麻粒岩和钙硅酸盐麻粒岩等组成。其中,含假蓝宝石泥质麻粒岩中石榴子石变斑晶和矽线石集合体(蓝晶石假象)周围分别发育峰期后由假蓝宝石+斜方辉石和假蓝宝石+堇青石后成合晶组成的典型减压结构。含石榴子石镁铁质麻粒岩中石榴子石变斑晶周围则发育峰期后由斜方辉石+斜长石后成合晶组成的典型白眼圈减压结构。不同研究者得出了具有不同超高温峰期条件、峰期前及峰期后演化历史、不同形式的顺时针变质P-T轨迹。对超高温变质事件发生的时间和构造背景的认识也存在较大分歧,有认为超高温变质事件发生于格林维尔期(~1000 Ma)并与碰撞造山和弧岩浆作用有关,也有研究认为发生于泛非期(~590 Ma或~530 Ma)并与普里兹造山及冈瓦纳大陆聚合有关。因此,为理清该区超高温麻粒岩的变质演化历史和构造背景,需要对其进一步进行详细深入的矿物组合-变质结构分析、P-T轨迹重建及高精度的锆石-独居石U-Pb年代学研究,并进行区域上对比。      

A curious case of agreement between conventional thermobarometry and phase equilibria modelling in granulites: New constraints on P–T estimates in the Antarctica segment of the Musgrave–Albany–Fraser–Wilkes Orogen

The Windmill Islands region in Wilkes Land, east Antarctica, preserves granulite facies metamorphic mineral assemblages that yield seemingly comparable P–T estimates from conventional thermobarometry and mineral equilibria modelling. This is uncommon in granulite facies terranes, where conventional thermobarometry and phase equilibria modelling generally produce conflicting P–T estimates because peak mineral compositions tend to be modified by retrograde diffusion processes. In situ U–Pb monazite geochronology and calculated metamorphic phase diagrams show that the Windmill Islands experienced two phases of high thermal gradient metamorphism during the Mesoproterozoic. The first phase of metamorphism is recorded by monazite ages in two widely separated samples and occurred at c. 1,305 Ma. This event was regional in extent, involved crustally derived magmatism and reached conditions of ~3.2–5 kbar and 690–770°C corresponding to very high thermal gradients of >150°C/kbar. The elevated thermal regime is interpreted to reflect a period of extension or increased extension in a back‐arc setting that existed prior to c. 1,330 Ma. The first metamorphic event was overprinted by granulite facies metamorphism at c. 1,180 Ma that was coeval with the intrusion of charnockite. This event involved peak temperatures of ~840–850°C and pressures of ~4–5 kbar. A phase of granitic magmatism at c. 1,250–1,210 Ma, prior to the intrusion of the charnockite, is interpreted to reflect a phase of compression within an overall back‐arc setting. Existing conventional thermobarometry suggests conditions of ~4 kbar and 750°C for M₁ and 4–7 kbar and 750–900°C for M₂. The apparent similarities between the phase equilibria modelling and existing conventional thermobarometry may suggest either that the terrane cooled relatively quickly, or that the P–T ranges obtained from conventional thermobarometry are sufficiently imprecise that they cover the range of P–T conditions obtained in this study. However, without phase equilibria modelling, the veracity of existing conventional P–T estimates cannot be evaluated. The calculated phase diagrams from this study allow the direct comparison of P–T conditions in the Windmill Islands with phase equilibria models from other regions in the Musgrave–Albany–Fraser–Wilkes Orogen. This shows that the metamorphic evolution of the Wilkes Land region is very similar to that of the eastern Albany–Fraser Orogen and Musgrave Province in Australia, and further demonstrates the remarkable consistency in the timing of metamorphism and the thermal gradients along the ~5,000 km strike length of this system.     

Ultra high temperature metamorphism of mafic granulites from South Altyn Orogen,West China: A result from the rapid exhumation of deeply subducted continental crust

The South Altyn orogen in West China contains ultra high pressure (UHP) terranes formed by ultra‐deep (>150–300 km) subduction of continental crust. Mafic granulites which together with ultramafic interlayers occur as blocks in massive felsic granulites in the Bashiwake UHP terrane, are mainly composed of garnet, clinopyroxene, plagioclase, amphibole, rutile/ilmenite, and quartz with or without kyanite and sapphirine. The kyanite/sapphirine‐bearing granulites are interpreted to have experienced decompression‐dominated evolution from eclogite facies conditions with peak pressures of 4–7 GPa to high pressure (HP)–ultra high temperature (UHT) granulite facies conditions and further to low pressure (LP)–UHT facies conditions based on petrographic observations, phase equilibria modelling, and thermobarometry. The HP–UHT granulite facies conditions are constrained to be 2.3–1.6 GPa/1,000–1,070°C based on the observed mineral assemblages of garnet+clinopyroxene+rutile+plagioclase+amphibole±quartz and measured mineral compositions including the core–rim increasing anorthite in plagioclase (X_An = 0.52–0.58), core–rim decreasing jadeite in clinopyroxene (X_Jd = 0.20–0.15), and TiO₂ in amphibole (Ti^M2/2 = 0.14–0.18). The LP–UHT granulite facies conditions are identified from the symplectites of sapphirine+plagioclase+spinel, formed by the metastable reaction between garnet and kyanite at <0.6–0.7 GPa/940–1,030°C based on the calculated stability of the symplectite assemblages and sapphirine–spinel thermometer results. The common granulites without kyanite/sapphirine are identified to record a similar decompression evolution, including eclogite, HP–UHT granulite, and LP–UHT granulite facies conditions, and a subsequent isobaric cooling stage. The decompression under HP–UHT granulite facies is estimated to be from 2.3 to 1.3 GPa at ~1,040°C on the basis of textural records, anorthite content in plagioclase (X_An = 0.25–0.32), and grossular content in garnet (X_Grs = 0.22–0.19). The further decompression to LP–UHT facies is defined to be >0.2–0.3 GPa based on the calculated stability for hematite‐bearing ilmenite. The isobaric cooling evolution is inferred mainly from the amphibole (Ti^M2/2 = 0.14–0.08) growth due to the crystallization of residual melts, consistent with a temperature decrease from >1,000°C to ~800°C at ~0.4 GPa. Zircon U–Pb dating for the two types of mafic granulite yields similar protolith and metamorphic ages of c. 900 Ma and c. 500 Ma respectively. However, the metamorphic age is interpreted to represent the HP–UHT granulite stage for the kyanite/sapphirine‐bearing granulites, but the isobaric cooling stage for the common granulites on the basis of phase equilibria modelling results. The two types of mafic granulite should share the same metamorphic evolution, but show contrasting features in petrography, details of metamorphic reactions in each stage, thermobarometric results, and also the meaning of zircon ages as a result of their different bulk‐rock compositions. Moreover, the UHT metamorphism in UHP terranes is revealed to represent the lower pressure overprinting over early UHP assemblages during the rapid exhumation of ultra‐deep subducted continental slabs, in contrast to the cause of traditional UHT metamorphism by voluminous heat addition from the mantle.     

胶北地体多期变质事件的P-T-t轨迹及其对胶-辽-吉带形成与演化的制约

胶北地体位于华北克拉通东部陆块胶-辽-吉带南端,主要由闪长质-TTG-花岗质片麻岩、变质表壳岩系和变质镁铁-超镁铁质岩所组成。本文通过对胶北早前寒武纪变质岩系的岩石学、矿物化学、变质反应结构和序列、变质温度和压力估算与同位素年代学资料的综合研究和总结,得出以下重要结论:(1)与华北克拉通东部陆块其它地区太古宙变质基底类似,本区也存在~2500Ma区域性新太古代变质事件,且与本区2550~2500Ma岩浆作用在时间上非常接近,其变质作用发生的时间比岩浆作用要晚10~50Myr,指示本区~2500Ma区域性变质事件可能与大规模的幔源岩浆底侵作用存在密切的成因关系。(2)胶北还存在1950~1850Ma区域性古元古代变质事件,并导致了大量高压基性和泥质麻粒岩的形成,高压基性麻粒岩主要以不规则透镜体、变形岩墙群或岩脉群的形式赋存于闪长质-TTG-花岗质片麻岩之中,并集中分布在安丘-平度-莱西-莱阳-栖霞一带,大致沿北东-南西向断续带状分布,构成了一条长约300km的古元古代高压麻粒岩相变质带。(3)本区古元古代高压麻粒岩以记录近等温减压(ITD)及随后近等压降温(IBC)的顺时针P-T-t轨迹为特征,指示本区变质杂岩在古元古代晚期曾强烈地卷入了与俯冲-拼贴-碰撞造山有关的构造过程,并可能经历了如下复杂的构造演化:(I)在古元古代晚期2000~1950Ma,随着有限大洋地壳的持续俯冲作用,本区各类变质岩的原岩开始经历一次构造增厚事件,并导致了它们的原岩经历了早期绿片岩相-角闪岩相进变质作用;(II)1950~1870Ma,大洋地壳俯冲作用结束,本区开始发生弧-陆拼贴和陆-陆碰撞作用,大陆地壳持续缩短和加厚,在加厚下地壳或岛弧根部带约50km的深度,发生了区域性高压麻粒岩相变质作用,并导致了本区变基性岩和变泥质岩分别形成了石榴石+单斜辉石+斜长石±角闪石±石英±铁-钛氧化物和石榴石+蓝晶石+钾长石+斜长石+黑云母+石英+铁-钛氧化物+熔体的高压麻粒岩相矿物组合。(III)1870~1800Ma,在同碰撞峰期变质结束之后,本区造山作用进入了后碰撞构造折返-伸展演化阶段,先后经历了早期快速构造折返和晚期缓慢冷却降温两个构造热演化阶段。其中,在早期快速构造折返阶段,高压麻粒岩经历了峰后近等温或略微增温减压退变质作用的叠加,高压基性麻粒岩表现为沿石榴石边部形成了含斜方辉石的后成合晶。与此同时,早期快速构造折返阶段还伴随着热松弛和伸展作用,出现一系列的幔源基性岩浆活动,不仅导致了本区大量未经历高压麻粒岩相变质的变基性岩群的形成,同时也诱发了区内大规模的地壳深熔作用的发生。自温度高峰期之后,本区地壳岩石还经历了一个近等压冷却降温过程,并发生了区域性角闪岩相退变质作用,高压基性麻粒岩表现为石榴石和斜方辉石边部常出现含角闪石的退变边或后成合晶。最终,在1800Ma左右,本区含电气石花岗伟晶质岩脉的大量出现,则标志着胶北地体古元古代晚期(2000~1800Ma)俯冲-拼贴-碰撞造山作用的最终结束。     

Single zircon evaporation ages: Evidence for the Mesoproterozoic crust in the southeastern Nigerian basement complex

The Ukwortung area of the Obudu Plateau exposes high-grade metamorphic rocks of upper amphibolite to granulite facies. Zircon grains from three locations of Southwest Obudu Plateau were dated by the Pb-Pb dating method. This single zircon radiometric dating method confirms the occurrence of Mesoproterozoic crust in the southeastern Nigeria basement complex. The pyroxene gneiss (granulite facies) sample from Biereberi near Ukpe yielded a mean age of 2061.4±0.4 Ma, suggesting the presence of Paleoproterozoic crust. The combined 207Pb/206Pb ratios from the garnet-biotite gneiss in the area gave a mean age of 1794.4±0.4 Ma. This further confirms the pres-ence of Paleoproterozoic crustal components within the Obudu Plateau. The Mesoproterozoic (1548.8±0.5 Ma) and Neoproterozoic (619.8±0.9 Ma) ages were obtained from two populations of zircon grains from the leucogranite in Okordem, Southwest Obudu. These "signatures" collaborate the occurrence of Mesoproterozoic ages in the Afar region and Cameroon basements along with the southeastern Nigerian basement complex, constituting the major portion of the West Central African mobile belt. The zircon age of 1794.4±0.4 Ma may probably be the maximum depositional age of the meta-sediment (garnet-biotite gneiss) and the age of 1548.8±0.5 Ma may be interpreted as the time of emplacement of the granitoid. The Neoproterozoic age (619.8±0.9 Ma) probably records the metamorphic event that was prevalent during the Pan-African period and thus affected the area.     

Metamorphism and exhumation of basement gneiss domes in the Quadrilátero Ferrífero:Two stage dome-and-keel evolution?

The presence of dome-and-keel provinces in Archean cratons has been connected with the initiation of plate tectonics on Earth as these features are most commonly observed in Archean rocks.The Quadrilátero Ferrífero in Brazil has been identified as a Paleoproterozoic dome-and-keel province for more than three decades.The prevailing model suggests that it formed during the Rhyacian Transamazonian orogeny,making it unique among dome-and-keel provinces.However,a lack of appropriate lithologies,datable minerals and the metamorphic overprint of later orogenesis has resulted in a cryptic metamorphic record for the formation of this dome-and-keel province.A clinopyroxene-bearing migmatite from the core of the Ba??o dome has peak P-T conditions of 5-7 kbar and 700-750 ℃ and a published age of ca.2730 Ma based on U-Pb ages of zircon from leucosomes,suggesting that this age represents the migmatisation event.A fine-grained epidote-albite-titanite assemblage overprints the coarse-grained clinopyroxene and amphibole,giving P-7 conditions of 8-9 kbar and 550 ℃ with an associated titanite age of ca.2050 Ma.A garnet-bearing amphibolite sample also from the core of the dome has peak P-T conditions of 7-8 kbar and 650-700 ℃,and texturally late titanite from this sample produces an age of ca.2060 Ma.Three additional samples were collected from the edges of the dome.A garnet-gedrite bearing felsic schist produces peak P-T conditions of 8-9 kbar and 650-700℃ on a clockwise P-T evolution.This sample has a U-Pb zircon age of ca.2775 Ma,which could date metamorphism or be the age of its volcaniclastic protolith.Texturally unconstrained titanite from the sample gives an age of ca.2040 Ma.A garnet-bearing amphibolite that occurs as a boudin within the felsic schist gives both zircon and titanite ages of ca.2050 Ma and has peak P-T conditions of 5-6 kbar and 650-700 ℃ on a near isobaric P-T path.An amphibolite dike,observed to cross-cut the felsic schist produces a zircon U-Pb age of ca.2760 Ma.Altogether this data suggests that the samples were metamorphosed in the Archean(ca.2775-2730 Ma)and again during the Transamazonian event.The most plausible explanation for this data is that dome-and-keel formation occurred in the Archean with migmatisation and high-temperature metamorphism occurring at this time.The Paleoproterozoic event is interpreted as a reactivation of the dome-and-keel formation structures,with Paleoproterozoic keels crosscutting Archean keels and producing metamorphic aureoles.The high radiogenic heat production and the presence of dense sedimentary successions in Archean terranes make dome-and-keel provinces a uniquely Archean feature,but they are susceptible to reworking,resulting in an enigmatic record of formation.     

http://www.sciencedirect.com/science/article/pii/S1674987112000564

High-pressure（HP） granulites widely occur as enclaves within tonalite-trondhjemitegranodiorite （TTG） gneisses of the Early Precambrian metamorphic basement in the Shandong Peninsula, southeast part of the North China Craton（NCC）.Based on cathodoluminescence（CL）,laser Raman spectroscopy and in-situ U-Pb dating,we characterize the zircons from the HP granulites and group them into three main types:inherited（magmatic） zircon,HP metamorphic zircon and retrograde zircon.The inherited zircons with clear or weakly defined magmatic zoning contain inclusions of apatites,and ²⁰⁷Pb/²⁰⁶Pb ages of 2915—2890 Ma and 2763—2510 Ma,correlating with two magmatic events in the Archaean basement. The homogeneous HP metamorphic zircons contain index minerals of high-pressure metamorphism including garnet,clinopyroxene.plagioclase,quartz,rutile and apatite,and yield ²⁰⁷Pb/²⁰⁶Pb ages between 1900 and 1850 Ma,marking the timing of peak HP granulite fades metamorphism.The retrograde zircons contain inclusions of orthopyroxene.plagioclase.quartz,apatite and amphibole.and yield the youngest ²⁰⁷Pb/²⁰⁶Pb ages of 1840—1820 Ma among the three groups,which we correlate to the medium to low-pressure granulite fades retrograde metamorphism.The data presented in this study suggest subduction of Meso- and Neoarchean magmatic protoliths to lower crust depths where they were subjected to HP granulite facies metamorphism during Palaeoproterozoic（1900—1850 Ma）.Subsequently, the HP granulites were exhumated to upper crust levels,and were overprinted by medium to low-pressure granulite and amphibolite facies retrograde event at ca.1840—820 Ma.     

柴北缘欧龙布鲁克地块东段古元古代基性麻粒岩:岩石学、锆石U-Pb年代学和Lu-Hf同位素证据

在欧龙布鲁克地块东部地区的正片麻岩中识别出呈透镜状产出的基性麻粒岩,部分已转变为斜长角闪岩。其主要矿物组合为单斜辉石、斜方辉石、斜长石、角闪石等,为典型中低压麻粒岩相组合。锆石SHRIMP U-Pb定年得到基性麻粒岩1928±9Ma的变质年龄,片麻岩围岩得到了1927±20Ma的变质年龄,以及2368±5Ma、2377±7Ma的岩浆结晶年龄。片麻岩锆石Hf同位素数据显示变质锆石及岩浆锆石均具有相似的Hf同位素成分,其二阶段模式年龄为2590~2830Ma,显示其可能源于太古代地壳物质的再造。欧龙布鲁克地块古元古代岩浆及变质演化历史与塔里木克拉通及华北克拉通很高的相似性,预示着在古元古代三者可能具有一定的亲缘性。     

辽东半岛南辽河群锆石U-Pb年代学及其地质意义

本文报道了辽东半岛古元古代胶-辽-吉活动带南辽河群中变质火山岩和沉积岩的锆石U-Pb年代学数据。变质流纹岩的锆石具典型的岩浆振荡环带结构和较高的Th/U比值(0.3),锆石U-Pb年龄为~2.2Ga,该年龄可代表其原岩形成年龄,在误差范围内与古元古代辽吉花岗岩年龄一致,表明辽吉花岗岩并不是辽河群的基底。变质玄武岩的锆石阴极发光强度较弱、弱分带或无分带,同时具较低的Th/U比值(0.1),为典型的变质成因锆石,锆石U-Pb年龄为~1.9Ga,代表其变质时代。变质沉积岩的碎屑锆石年龄主要介于1981~3520Ma之间:峰期年龄为2033Ma和2092Ma的锆石年龄信息暗示辽东半岛至少存在一期2000~2100Ma的岩浆事件,并且该时期的中酸性岩浆岩是南辽河群沉积岩的一个重要物源;峰期年龄为2155Ma、2446Ma、2509Ma、2594Ma、2668Ma、2790Ma、3356Ma、3467Ma和3520Ma的锆石年龄信息,区域上与古元古代辽吉花岗岩、辽河群火山岩及太古宙基底年龄相吻合,暗示它们为南辽河群沉积岩提供了重要物源。沉积岩中最年轻的碎屑锆石U-Pb年龄为~2.0Ga,可代表其沉积时的最大年龄。所以,辽河群火山-沉积-变质的时限为2.2~1.9Ga,其演化时间约300Myr。结合前人有关辽东半岛前寒武纪岩石的研究成果,本文研究认为胶-辽-吉活动带的形成演化与弧-陆碰撞有关,而不是许多人坚持的裂谷环境。