The Kabul Block (Afghanistan), a segment of the Columbia Supercontinent,with a Neoproterozoic metamorphic overprint

We report field relationships, petrography and isotopic ages from two superposed basement units of the Kabul Block, the so called Lower Sherdarwaza and Upper Welayati formations. The Sherdarwaza Formation is represented mostly by migmatites and gneisses that are derived from pelitic and psammitic lithologies with lenses and layers of mafic and carbonate rocks. Several bodies of orthogneisses are also exposed in the Sherdarwaza Formation. The Upper Welayati Formation is characterized by micaschist, quartzite and amphibolites. SHRIMP U–Pb data on zircon from the orthogneiss in the Sherdarwaza Formation indicates a Neoarchean age of ca 2.5–2.8 Ga for their magmatic crystallization. The rocks exhibit granulite facies conditions of 5–7 kbar and 800 °C that are documented by the presence of orthopyroxene and Ti-rich biotite in the orthogneiss and by olivine and phlogopite in some calc-silicate rocks at contact with marble. A Paleoproterozoic age of ca. 1.85–1.80 Ga for this metamorphism was obtained using U-Pb SHRIMP dating on zircon and U-Th dating on monazite. Mineral textural relations also show a younger amphibolite facies metamorphism that is documented in both the Sherdarwaza and Welayati formations. This metamorphism occurred at relatively higher pressure conditions of up to 9 kbar at ca. 650 °C, compared to the granulite facies event. A Neoproterozoic age of ca 0.85–0.9 Ga, for this metamorphism is confirmed by Ar-Ar data on biotite and white mica as well as by U-Th data on monazite. By combining the presented results on the metamorphic petrology, geochronology and geochemistry, we conclude that: (1) The Kabul basement is a fragment of an Archean block (craton); (2) the ca. 1.85–1.8 and 0.9–0.85 Ga metamorphism marks an important orogenic events for the basement rocks of the Kabul Block which was stabilized during the early Precambrian; (3) the two metamorphic ages correlate well with global-scale orogenies related to the assembly of the Paleoproterozoic Columbia and Neoproterozoic Rodinia supercontinents; (4) based on metamorphic characteristics and ages, the Kabul basement rocks show an affinity to the Neoarchean rocks of the Tarim and/or South China cratons.     

大别山北大别杂岩的大地构造属性

北大别杂岩主要由花岗质片麻岩及斜长角闪岩组成 ,含有不同类型、大小不等的麻粒岩岩块和变质超镁铁质岩块 ,侵入有大量白垩纪花岗岩和辉石 -辉长岩类。其中的花岗质片麻岩、斜长角闪岩具有岛弧环境的岩石地球化学特征 ,代表拼贴于扬子陆块北缘的新元古代古岛弧。北大别杂岩北可与庐镇关群相连 ,南俯于超高压变质岩之下 ,在三叠纪扬子陆块与华北陆块的碰撞过程中 ,曾与超高压变质岩一起俯冲到地幔深度并经受榴辉岩相变质作用 ,然后在折返过程中叠加了麻粒岩相及角闪岩相变质作用 ,是扬子陆块北缘陆壳俯冲基底的一部分     

Building the Hindu Kush: monazite records of terrane accretion,plutonism and the evolution of the Himalaya–Karakoram–Tibet orogen

In situ U‐Th/Pb (LA‐ICP‐MS) monazite ages from the Hindu Kush of NW Pakistan provide new petrochronologic constraints on the tectonic evolution of the Himalaya–Karakoram–Tibet orogen. Monazites from two adjacent garnet + staurolite schist specimens yield multiple age populations that record the major Mesozoic and Cenozoic deformational, magmatic and metamorphic events along the southern margin of Eurasia. These include the accretion of the Hindu Kush–SW Pamir to Eurasia during the Late Triassic, followed by the accretion of the Karakoram terrane in the Early Jurassic. Younger Jurassic and Cretaceous ages record the development of an Andean‐style volcanic arc along the southern Eurasian margin, which ended with the docking of the Kohistan island arc and the emplacement of the Kohistan–Ladakh batholith during the Late Cretaceous. The initial Eocene collision of India with Eurasia was followed by widespread high‐temperature metamorphism and anatexis associated with crustal thickening within the Himalaya system in the Late Oligocene and Early Miocene.     

贺兰山——阿拉善地区下、中前寒武系的划分对比及其变质、成矿作用特征

据近期成果,贺兰山—阿拉善地区出露的巨厚变质杂岩可划分为中太古界贺兰山群和叠布斯格群(其全岩Rb—Sr等时年龄为3108.3和3218.8Ma),上太古界阿拉善群和下元古界的赵池沟群、阿拉坦敖包群;它们具不同的变质矿物共生组合,太古界变质岩属低压高温变质的麻粒岩相;下元古界为低—低中压区域动力(热流)变质的绿片岩相岩石。太古界有较强的混合岩化、花岗岩化作用,并蕴藏有铁、石墨、矽线石、刚玉等多种矿产。     

Two types of Precambrian high-grade metamorphism, Inner Mongolia, China

Abstract Archaean and Proterozoic granulite facies complexes of Inner Mongolia differ in lithological association, tectonic style, mineral assemblage and metamorphic P–T path. A nearly isobaric cooling path for Archaean high-grade metamorphic rocks is suggested by reaction textures and geothermobarometry. Early Proterozoic metamorphic rocks show nearly isothermal decompression. Archaean metamorphism may have been caused by magmatic accretion, whereas early Proterozoic metamorphism suggests a major continental thickening event followed by exhumation.     

Geochronology and geochemistry of metamorphic rocks in the Jiaobei terrane: Constraints on its tectonic affinity in the Sulu orogen

Tectonic affinity of tectono-lithological units close to ultrahigh-pressure metamorphic belt is a key issue for understanding the geodynamics of continental collision. This is particularly so for the Jiaobei terrane northeast of the Dabie-Sulu orogenic belt in China. New data from LA-ICPMS zircon U–Pb dating, whole-rock elements and Nd–Sr isotopes, and mineral O isotopes are presented for metamorphic rocks from this terrane. The results place geochronological and geochemical constraints on their protolith nature and metamorphic timing and thus on its tectonic affinity to one of the two Triassic collided continents, the North and the South China Blocks. Protolith ages for TTG gneiss, amphibolite and mafic granulite are ∼2.7, ∼2.5 and ∼2.4 Ga, respectively; regional metamorphism took place extensively at ∼1.76 Ga. Protolith of the TTG gneiss was generated by partial melting of mantle-derived rocks at the root of a thickened crust. Protolith of the amphibolite was probably a product of arc-like magmatism; protolith of the mafic granulite was derived from a depleted mantle source. Both of them were locally contaminated by supracrustal materials. Protoliths of paragneiss and schist in the Fenzishan Group were mostly derived from supracrustal sources, but protolith of amphibolite in the Fenzishan Group is of mantle-derived signature. Unlike the UHP metaigneous rocks in the Dabie-Sulu orogenic belt that show unusual ¹⁸O-depletion, the Jiaobei metamorphic rocks have basically preserved their original mantle-like O isotope compositions. In general, the nature and timing of geological events recorded in the metamorphic rocks from the Jiaobei terrane are comparable with those from the North China Block rather than the South China Block. Thus, the Jiaobei terrane is concluded to have tectonic affinity to the former, but behave like a micro-continent during the Triassic continental collision. The ∼1.76 Ga regional metamorphism in the Jiaobei terrane is likely related to reworking of the arc-continent collisional orogen in the periphery of the North China Block rather than the ∼1.85 Ga collision event between the eastern and western North China Blocks. The present study lends support to the common assumption that the suture boundary between the North and South China Blocks in the Sulu orogen is located along the Wulian-Yantai fault. Tectonic mingling along the Wulian-Yantai fault is probably related to subduction erosion during the continental collision.     

SHRIMP U–Pb zircon dating of the host rocks of the Cannington Ag–Pb–Zn deposit,southeastern Mt Isa Block,Australia

SHRIMP U–Pb zircon ages are reported from a paragneiss, a pegmatite, a metasomatised metasediment and an amphibolite taken from the upper amphibolite facies host sequence of the Cannington Ag–Pb–Zn deposit at the southeastern margin of the Proterozoic Mt Isa Block. Also reported are ages from a middle amphibolite‐facies metasediment from the Soldiers Cap Group approximately 90 km north of Cannington. The predominantly metasedimentary host rocks of the Cannington deposit were eroded from a terrane containing latest Archaean to earliest Palaeoproterozoic (ca 2600–2300 Ma) and Palaeoproterozoic (ca 1750–1700 Ma) zircon. The ca 1750–1700 Ma group of zircons are consistent with sedimentary provenance from rocks of Cover Sequence 2 age that are now exposed to the north and west of the Cannington deposit. The metasedimentary samples also include a group of zircon grains at ca 1675 Ma, which we interpret as the maximum depositional age of the sedimentary protolith. This is comparable to the maximum depositional age of the metasediment from the Maronan area (ca 1665 Ma) and to previously published data from the Soldiers Cap Group. Metamorphic zircon rims and new zircon grains grew at 1600–1580 Ma during upper amphibolite‐facies metamorphism in metasedimentary and mafic magmatic rocks. Zircon inheritance patterns suggest that sheet‐like pegmatitic intrusions were most likely derived from partial melting of the surrounding metasediments during this period of metamorphism. Some zircon grains from the amphibolite have a morphology consistent with partially recrystallised igneous grains and have apparent ages close to the metamorphic age, although it is not clear whether these represent metamorphic resetting or crystallisation of the magmatic protolith. Pb‐loss during syn‐ to post‐metamorphic metasomatism resulted in partial resetting of zircons from the metasomatised metasediment.     

Granulites of the Kolpakovskaya Series in the sredinny range,Kamchatka: A myth or reality?

Geological, mineralogical, and geothermobarometric data testify that the regional metamorphism of the terrigenous protolith of the Kolpakovskaya Series, which composes the stratigraphic basement of the Kamchatka Median Crystalline Massif, corresponded to the kyanite mineral subfacies of the amphibolite facies at temperatures of 560–660°C and pressures of 5.9–6.9 kbar. This metamorphism predetermined wide kyanite development in high-Al garnet-biotite plagiogneisses. The younger granitization and migmatization of the plagiogneisses took place at a decrease in the pressure (depth), as follows from the textures of kyanite reaction replacement by andalusite in both the metamorphic rocks and the vein synmetamorphic granitoids and pegmatites. The temperature of the granitization and migmatization processes in the plagiogneisses was estimated at 620–650°C, and the pressure was evaluated at 1.9–3.0 kbar. Acid leaching that accompanied the granitization and migmatization processes resulted in the intense replacement of biotite by sillimanite (fibrolite) and, to a lesser degree, muscovite in the metamorphic and vein magmatic rocks. The highest temperature orthopyroxene-cordierite-biotite-orthoclase-plagioclase-quartz mineral assemblages were determined to have been formed in the Kolpakovskaya Series at a temperature of 830–840°C not by the regional metamorphism but in contact aureoles around gabbro-granitoid intrusions of the Lavkinskii intrusive complex of Oligocene-Miocene age in garnet-biotite and kyanite-garnet-biotite plagiogneisses of the amphibolite facies and cannot thus be regarded as evidence of an early granulite stage in the metamorphism of these rocks.     

SHRIMP U-Pb zircon dating from Sulu-Dabie dolomitic marble, eastern China: constraints on prograde, ultrahigh-pressure and retrograde metamorphic ages

Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircon from Sulu‐Dabie dolomitic marbles is characterized by distinctive domains of inherited (detrital), prograde, ultrahigh‐pressure (UHP) and retrograde metamorphic growths. The inherited zircon domains are dark‐luminescent in CL images and contain mineral inclusions of Qtz + Cal + Ap. The prograde metamorphic domains are white‐luminescent in CL images and preserve a quartz eclogite facies assemblage of Qtz + Dol + Grt + Omp + Phe + Ap, formed at 542–693 °C and 1.8–2.1 GPa. In contrast, the UHP metamorphic domains are grey‐luminescent in CL images, retain the UHP assemblage of Coe + Grt + Omp + Arg + Mgs + Ap, and record UHP conditions of 739–866 °C and >5.5 GPa. The outermost retrograde rims have dark‐luminescent CL images, and contain low‐P minerals such as calcite, related to the regional amphibolite facies retrogression. Laser ablation ICP‐MS trace‐element data show striking difference between the inherited cores of mostly magmatic origin and zircon domains grown in response to prograde, UHP and retrograde metamorphism. SHRIMP U‐Pb dating on these zoned zircon identified four discrete ²⁰⁶Pb/²³⁸U age groups: 1823–503 Ma is recorded in the inherited (detrital) zircon derived from various Proterozoic protoliths, the prograde domains record the quartz eclogite facies metamorphism at 254–239 Ma, the UHP growth domains occurred at 238–230 Ma, and the late amphibolite facies retrogressive overprint in the outermost rims was restricted to 218–206 Ma. Thus, Proterozoic continental materials of the Yangtze craton were subducted to 55–60 km depth during the Early Triassic and recrystallized at quartz eclogite facies conditions. Then these metamorphic rocks were further subducted to depths of 165–175 km in the Middle Triassic and experienced UHP metamorphism, and finally these UHP metamorphic rocks were exhumed to mid‐crustal levels (about 30 km) in the Late Triassic and overprinted by regional amphibolite facies metamorphism. The subduction and exhumation rates deduced from the SHRIMP data and metamorphic P–T conditions are 9–10 km Myr^?1 and 6.4 km Myr^?1, respectively, and these rapid subduction–exhumation rates may explain the obtained P–T–t path. Such a fast exhumation suggests that Sulu‐Dabie UHP rocks that returned towards crustal depths were driven by buoyant forces, caused as a consequence of slab breakoff at mantle depth.     

新疆前震旦纪地层划分及地壳演化

根据变质地质学的观点,新疆的前震旦纪地层可划分为太古界、下元古界和中一上无古界,它们在岩石组合、原岩建造、变质作用类型及形成环境等方面具有明显的区别,同时表明古西北陆台具有与古华北陆台完全不同的地质发展历史。     

点苍山变质杂岩新生代变质-变形演化及其区域构造内涵

点苍山变质杂岩体是哀牢山-红河韧性剪切带四个变质杂岩体之一,遭受了多期多阶段变质-变形作用改造。本文重点针对点苍山杂岩的新生代变质-变形作用,尤其是以富铝质高级变质岩即夕线石榴黑云片麻岩和侵位于其中的糜棱岩化细晶花岗质岩石开展了深入研究。对夕线石榴黑云片麻岩的显微构造分析与矿物共生组合研究,确定了高角闪岩相和低角闪岩相变质矿物共生组合,分别为:石榴石(Grt)+夕线石(Sil)+钾长石(Kfs)+黑云母(Bi)+斜长石(Pl)±石英(Q)和夕线石(Sil)+白云母(Ms)+黑云母(Bi)+石英(Q)。对其中的变质锆石进行SHRIMP U-Pb测试,获得了新生代三个阶段的变质作用年龄,即54.2±1.7Ma、31.5±1.5Ma和27.5±1.2Ma.本文还深入研究了侵位于高级变质岩中的一个花岗岩质糜棱岩的宏观与显微构造特点,其LA-ICP-MS年龄为24.4±0.89Ma,代表着同剪切就位花岗质岩浆侵位和结晶年龄。夕线石榴黑云片麻岩中变质锆石从2150～27Ma多期多阶段表观年龄的发育,表明点苍山变质杂岩体具有复杂的构造演化史。点苍山杂岩的多阶段新生代构造-热演化归咎于印度-欧亚板块会聚与碰撞作用(约54Ma)、造山后伸展作用(大约40～30Ma)和沿着哀牢山-红河剪切带大规模左行走滑变形作用(约27～21Ma)。     

Geochronological constraints on the timing of granitoid magmatism, metamorphism and post-metamorphic cooling in the Hercynian crustal cross-section of Calabria

Exposed cross‐sections of the continental crust are a unique geological situation for crustal evolution studies, providing the possibility of deciphering the time relationships between magmatic and metamorphic events at all levels of the crust. In the cross‐section of southern and northern Calabria, U–Pb, Rb–Sr and K–Ar mineral ages of granulite facies metapelitic migmatites, peraluminous granites and amphibolite facies upper crustal gneisses provide constraints on the late‐Hercynian peak metamorphism and granitoid magmatism as well as on the post‐metamorphic cooling. Monazite from upper crustal amphibolite facies paragneisses from southern Calabria yields similar U–Pb ages (295–293±4 Ma) to those of granulite facies metamorphism in the lower crust and of intrusions of calcalkaline and metaluminous granitoids in the middle crust (300±10 Ma). Monazite and xenotime from peraluminous granites in the middle to upper crust of the same crustal section provide slightly older intrusion ages of 303–302±0.6 Ma. Zircon from a mafic to intermediate sill in the lower crust yields a lower concordia intercept age of 290±2 Ma, which may be interpreted as the minimum age for metamorphism or intrusion. U–Pb monazite ages from granulite facies migmatites and peraluminous granites of the lower and middle crust from northern Calabria (Sila) also point to a near‐synchronism of peak metamorphism and intrusion at 304–300±0.4 Ma. At the end of the granulite facies metamorphism, the lower crustal rocks were uplifted into mid‐crustal levels (10–15 km) followed by nearly isobaric slow cooling (c. 3 °C Ma^?1) as indicated by muscovite and biotite K–Ar and Rb–Sr data between 210±4 and 123±1 Ma. The thermal history is therefore similar to that of the lower crust of southern Calabria. In combination with previous petrological studies addressing metamorphic textures and P–T conditions of rocks from all crustal levels, the new geochronological results are used to suggest that the thermal evolution and heat distribution in the Calabrian crust were mainly controlled by advective heat input through magmatic intrusions into all crustal levels during the late‐Hercynian orogeny.     

Petrogenesis of Eclogites in the Light of Punctuated Metamorphic Evolution in Dabie Terrane，China

Ｐｅｔｒｏｇｅｎｅｓｉｓ　ｏｆ　Ｅｃｌｏｇｉｔｅｓ　ｉｎ　ｔｈｅ　Ｌｉｇｈｔ　ｏｆ　ＰｕｎｃｔｕａｔｅｄＭｅｔａｍｏｒｐｈｉｃ　Ｅｖｏｌｕｔｉｏｎ　ｉｎ　Ｄａｂｉｅ　Ｔｅｒｒａｎｅ，Ｃｈｉｎａ￥ＹｏｕＺｈｅｎｄｏｎｇ；ＨａｎＹｕｊｉｎｇ；ＺｈｏｎｇＺ...     

Relics of the Mozambique Ocean in the central East African Orogen: evidence from the Vohibory Block of southern Madagascar

The Vohibory Block of south‐western Madagascar is part of the East African Orogen, the formation of which is related to the assembly of the Gondwana supercontinent. It is dominated by metabasic rocks, which have chemical compositions similar to those of recent basalts from a mid‐ocean ridge, back‐arc setting and island‐arc setting. The age of formation of protolith basalts has been dated at 850–700 Ma by U–Pb SHRIMP analysis of magmatic cores in zircon, pointing to an origin related to the Neoproterozoic Mozambique Ocean. The metabasic rocks are interpreted as representing components of an island arc with an associated back‐arc basin. In the early stage of the Pan‐African orogeny, these rocks experienced high‐pressure amphibolite to granulite facies metamorphism (9–12 kbar, 750–880 °C), dated at 612 ± 5 Ma from metamorphic rims in zircon. The metamorphism was most likely related to accretion of the arc terrane to the margin of the Azania microcontinent (Proto‐Madagascar) and closure of the back‐arc basin. The main metamorphism is significantly older than high‐temperature metamorphism in other tectonic units of southern Madagascar, indicating a distinct tectono‐metamorphic history.     

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.     

中国与蒙古之地质

东昆仑中部缝合带清水泉一带发育石榴斜长紫苏麻粒岩、紫苏辉石黑云母石榴子石麻粒岩、石榴二辉斜长麻粒岩和石榴单斜辉石麻粒岩,它们与混合岩化黑云母石榴子石变粒岩、黑云母辉石变粒岩、石墨大理岩、含透辉石透闪石大理岩、透辉石大理岩、黑云斜长角闪岩和片麻岩等高级变质岩系以及纯橄岩、辉橄岩、橄长岩、辉长岩、辉绿岩和玄武岩等共同构成蛇绿混杂岩。麻粒岩相变质作用的温压条件为T=760~880℃,p=830~1200MPa,为高温中高压麻粒岩相变质作用,估算其形成深度为40~45km。麻粒岩相变质作用的SHRIMP锆石U-Pb年龄为(507·7±8·3)Ma。清水泉地区蛇绿岩形成于~520Ma,到~508Ma时俯冲至地下40~45km深处而发生中高压麻粒岩相变质作用,然后发生构造折返而剥露至地表。证实了清水泉高级变质岩和基性—超基性岩片是形成于早—中寒武世的蛇绿混杂岩,标志一个古生代早期的非常重要的板块汇聚边界,这对于进一步研究东昆仑造山带构造演化、乃至中国西部大地构造格局具有非常重要的意义。     

Xes-Xen thermochronology of the Rayner metamorphic complex,Enderby Land (East Antarctica,Molodezhnaya Station Area)

The Xe_s-Xe_n dating of zircons from rocks of the Rayner Complex of the Enderby Land at the Molodezhnaya Station area (coast of the Alasheyev Bight) yielded age estimates of 550 ± 50 and 1040 ± 30 Ma. The metamorphic rocks of the Rayner Complex record two main events: first, the crystallization of the magmatic protoliths of charnockitic and enderbitic gneisses and, second, superimposed structural and metamorphic alterations under conditions transitional from the amphibolite to granulite facies (metamorphism manifested regionally in the rocks of the Rayner Complex). The most reliable Xe_s-Xe_n age estimates for magmatic zircons from the charnockitic and enderbitic gneisses correspond to the Grenville stage of the development of the Rayner Complex (～1.0 Ga). The Xe isotopic systematics of metamorphic zircons reflect a pan-African stage in the evolution of the Rayner Complex (600–550 Ma). Pan-African events are reflected in the U-Xe isotope system in two cases: if metamorphic zircons crystallized at the same time (which probably resulted in the formation of a plateau in the Xe_s-Xe_n age spectrum) and if the initial isotopic systems were disturbed (which resulted in a decrease in apparent age toward low-temperature gas fractions). It is important that secondary alterations and a decrease in apparent ages to 600–550 Ma affected only those components (i.e., caused xenon release only from those traps) that were unstable under the maximum metamorphic temperatures and yielded T _cl values lower than 750°C (conditions transitional from the amphibolite to granulite facies). At a higher xenon retention, “primary” isotopic systems are preserved. Consequently, the age of metamorphism transitional between the amphibolite and granulite facies can be estimated at 600–550 Ma on the basis of Xe_s-Xe_n dating. In general, the results of our study indicate that the age of regional metamorphism of the Rayner complex at the Molodezhnaya area is approximately 600–550 Ma rather than ～1.0 Ga, as was previously supposed.     

Magmatic arc metamorphism: petrology and temperature history of metabasic rocks in the Coastal Cordillera of northern Chile

We investigated the metamorphic cooling history of underplated magmatic rocks at midcrustal depth. Granulites and amphibolites occur within the Jurassic magmatic belt of the Coast Range south of Antofagasta in northern Chile between 23°25' and 24°20' S. The protoliths of the metamorphic rocks are basic intrusions of Early Mesozoic age. They are part of the magmatically formed crust, and the essentially dry magmas were emplaced in an extensional regime. The granulites (clinopyroxene–orthopyroxene–plagioclase) show all stages of fabric development from magmatic to granoblastic fabrics. Pyroxene compositions were reset at temperatures around 800°  C independent of the stage of textural equilibration. The granulites were partially amphibolitized at upper amphibolite facies temperatures of 600–700°  C. Following cooling, a possible reheating to greenschist facies temperatures around 500°  C is indicated by prograde zoning in magnetite–ilmenite pairs. Mineral assemblages are not suitable for barometry, but a conservative estimation of the garnet-in reaction at given whole-rock compositions suggests maximum pressures in the granulite facies of around 5 kbar, and similar pressures are indicated by phengite barometry for the greenschist facies. The P–T  path of granulite–amphibolite metamorphism is one of slow cooling from magmatic temperatures with heterogeneous deformation. The thinning of the pre-Andean (Precambrian–Triassic) crust was apparently compensated by the magmatic underplating and this special tectonomagmatic setting caused the prolonged residence of the accreted rocks at midcrustal levels.     

Ultrahigh-pressure eclogite transformed from mafic granulite in the Dabie orogen, east-central China

Although ultrahigh‐pressure (UHP) metamorphic rocks are present in many collisional orogenic belts, almost all exposed UHP metamorphic rocks are subducted upper or felsic lower continental crust with minor mafic boudins. Eclogites formed by subduction of mafic lower continental crust have not been identified yet. Here an eclogite occurrence that formed during subduction of the mafic lower continental crust in the Dabie orogen, east‐central China is reported. At least four generations of metamorphic mineral assemblages can be discerned: (i) hypersthene + plagioclase ± garnet; (ii) omphacite + garnet + rutile + quartz; (iii) symplectite stage of garnet + diopside + hypersthene + ilmenite + plagioclase; (iv) amphibole + plagioclase + magnetite, which correspond to four metamorphic stages: (a) an early granulite facies, (b) eclogite facies, (c) retrograde metamorphism of high‐pressure granulite facies and (d) retrograde metamorphism of amphibolite facies. Mineral inclusion assemblages and cathodoluminescence images show that zircon is characterized by distinctive domains of core and a thin overgrowth rim. The zircon core domains are classified into two types: the first is igneous with clear oscillatory zonation ± apatite and quartz inclusions; and the second is metamorphic containing a granulite facies mineral assemblage of garnet, hypersthene and plagioclase (andesine). The zircon rims contain garnet, omphacite and rutile inclusions, indicating a metamorphic overgrowth at eclogite facies. The almost identical ages of the two types of core domains (magmatic = 791 ± 9 Ma and granulite facies metamorphic zircon = 794 ± 10 Ma), and the Triassic age (212 ± 10 Ma) of eclogitic facies metamorphic overgrowth zircon rim are interpreted as indicating that the protolith of the eclogite is mafic granulite that originated from underplating of mantle‐derived magma onto the base of continental crust during the Neoproterozoic (c. 800 Ma) and then subducted during the Triassic, experiencing UHP eclogite facies metamorphism at mantle depths. The new finding has two‐fold significance: (i) voluminous mafic lower continental crust can increase the average density of subducted continental lithosphere, thus promoting its deep subduction; (ii) because of the current absence of mafic lower continental crust in the Dabie orogen, delamination or recycling of subducted mafic lower continental crust can be inferred as the geochemical cause for the mantle heterogeneity and the unusually evolved crustal composition.     

U–Pb zircon study of tectonically bounded blocks of 2940–2840 Ma crust with different metamorphic histories,Paamiut region,South-West Greenland: implications for the tectonic assembly of the North Atlantic craton

New fieldwork, map interpretation, petrography and single zircon U–Pb geochronology has allowed the identification of different crustal blocks in the Paamiut region, in the southern portion of the West Greenland Archaean Craton. Changes of metamorphic grade from only amphibolite facies to granulite facies (some subsequently retrogressed) corresponds with zones of Archaean high strain ductile deformation ± mylonites. U–Pb zircon dates are presented for the TTG (tonalite, trondhjemite, granodiorite) protoliths from each block in the Paamiut region, and the southern portion of the previously identified Tasiusarsuaq terrane lying to the north. The southern part of the Tasiusarsuaq terrane contains 2880–2860 Ma TTG rocks and underwent amphibolite facies metamorphism. Structurally underneath the Tasiusarsuaq terrane to the south is the Sioraq block containing 2870–2830 Ma TTG rocks partly retrogressed from granulite facies. Structurally underneath and to the south is the Paamiut block, dominated by 2850–2770 Ma granodioritic rocks that have only undergone amphibolite facies metamorphism. Also structurally overlying the Paamiut block, but cropping out separately from the Sioraq block, is the Neria block. This appears to be dominated by 2940–2920 Ma gneisses that have been totally retrogressed from granulite facies and strongly deformed. In the southernmost part of the region the Neria block overlies the greenschist to lowermost amphibolite facies Sermiligaarsuk block that contains the ⩾2945 Ma Tartoq Group. Rocks from all the blocks record ancient loss of Pb from zircons and some new zircon growth at 2820 Ma, interpreted to indicate a high grade metamorphic event at that time, including granulite facies metamorphism in the Sioraq and Neria blocks. The blocks of different metamorphic grade are interpreted to have moved to their current positions after the 2820 Ma metamorphism, explaining the change in metamorphic history across some mylonites and ductile shear zones which deform and retrogress granulite facies textures. The juxtaposed blocks and their contacts were subsequently folded under amphibolite facies conditions. The contacts are cut by undeformed Palaeoproterozoic dolerite dykes which post-date amphibolite facies metamorphism. These results, together with previously published data from the Godthåbsfjord region (north of Paamiut) shows that the North Atlantic Craton in West Greenland from Ivittuut in the south to Maniitsoq in the north (∼550 km) consists of a mosaic of ductile fault-bounded packages that attained their present relative positions in the late Archaean.