晋南中条山古元古代花岗岩的锆石U—Pb年龄

用单颗粒锆石U-Pb稀释法测定了中条山古元古代花岗岩的时代,获得寨子英云闪长质片麻岩、横岭关花岗闪长质片麻岩和烟庄钾长-二长花岗质片麻岩的成岩年龄分别为2321 Ma±2Ma、2256Ma±35Ma和2297Ma±21 Ma.结合区域地质背景和地球化学资料,认为中条山地区可能存在时限为2350～2250Ma的构造-热事件.     

What Happened in the Trans-North China Orogen in the Period 2560-1850 Ma？

The Trans-North China Orogen （TNCO） was a Paleoproterozic continent-continent collisional belt along which the Eastern and Western Blocks amalgamated to form a coherent North China Craton （NCC）. Recent geological, structural, geochemical and isotopic data show that the orogen was a continental margin or Japan-type arc along the western margin of the Eastern Block, which was separated from the Western Block by an old ocean, with eastward-directed subduction of the oceanic lithosphere beneath the western margin of the Eastern Block. At 2550-2520 Ma, the deep subduction caused partial melting of the medium-lower crust, producing copious granitoid magma that was intruded into the upper levels of the crust to form granitoid plutons in the low- to medium-grade granite-greeustone terranes. At 2530-2520 Ma, subduction of the oceanic lithosphere caused partial melting of the mantle wedge, which led to underplating of mafic magma in the lower crust and widespread mafic and minor felsic volcanism in the arc, forming part of the greenstone assemblages. Extension driven by widespread mafic to felsic volcanism led to the development of back-arc and/or intra-arc basins in the orogen. At 2520-2475 Ma, the subduction caused further partial melting of the lower crust to form large amounts of tonalitic-trondhjemitic-granodioritic （TTG） magmatism. At this time following further extension of back-arc basins, episodic granitoid magmatism occurred, resulting in the emplacement of 2360 Ma, -2250 Ma 2110-21760 Ma and -2050 Ma granites in the orogen. Contemporary volcano-sedimentary rocks developed in the back-arc or intra-are basins. At 2150-1920 Ma, the orogen underwent several extensional events, possibly due to subduction of an oceanic ridge, leading to emplacement of mafic dykes that were subsequently metamorphosed to amphibolites and medium- to high-pressure mafic granulites. At 1880-1820 Ma, the ocean between the Eastern and Western Blocks was completely consumed by subduction, and the dosing of the ocean led to the continent-arc-continent collision, which caused large-scale thrusting and isoclinal folds and transported some of the rocks into the lower crustal levels or upper mantle to form granulites or eclogites. Peak metamorphism was followed by exhumation/uplift, resulting in widespread development of asymmetric folds and symplectic textures in the rocks.     

Petrological and geochronological constraints on the origin of the Palimé–Amlamé granitoids (South Togo, West Africa): A segment of the West African Craton Paleoproterozoic margin reactivated during the Pan-African collision

The Palimé–Amlamé Pluton (PAP) in southern Togo, consists of silica-rich to intermediate granitoids including enclaves of mafic igneous rocks and of gneisses. They are commonly called the “anatectic complex of Palimé–Amlamé” and without any convincing data, they were interpreted either as synkinematic Pan-African granitoids or as reworked pre Pan-African plutons. New field and petrological observations, mineral and whole-rock chemical analyses together with U–Pb zircon dating, have been performed to evaluate the geodynamic significance of the PAP within the Pan-African orogenic belt. With regard to these new data, the granitoids and related enclaves probably result from mixing and mingling processes between mafic and silicic magmas from respectively mantle and lower crust sources. They display Mg–calc-alkaline chemical features and present some similarities with Late Archaean granites such as transitional (K-rich) TTGs and sanukitoids.

The 2127 ± 2 Ma age obtained from a precise U/Pb concordia on zircon, points out a Paleoproterozoic age for the magma crystallization and a lower intercept at 625 ± 29 Ma interpreted as rejuvenation during Pan-African tectonics and metamorphism. Based on these results, a Pan-African syn to late orogenic setting for the PAP, i.e. the so-called “anatectic complex of Palimé–Amlamé”, can be definitively ruled out. Moreover according to its location within the nappe pile and its relationships with the suture zone, the PAP probably represents a fragment of the West African Craton reactivated during the Pan-African collision.     

Mineral chemistry constraints on the evolution of the 1.88–1.87 Ga post-kinematic granite plutons in the Central Finland Granitoid Complex

A suite of post-kinematic, 1.88–1.87 Ga, silicic plutons crosscut 1.89–1.88 Ga synkinematic granitoids in the Central Finland Granitoid Complex (CFGC) in south-central Finland. The plutons range from biotite±hornblende quartz monzonite to syenogranite and include pyroxene- and olivine-bearing varieties. Mineral chemical data on feldspars, biotite, amphibole, pyroxenes, olivine, and oxides of the post-kinematic plutons are presented. The data are interpreted to show that these plutons register (1) a considerable range in pressure from 2–4 kbar (amphibole barometry) to 5–7 kbar (olivine–pyroxene barometry), (2) temperatures mostly reflecting resetting during cooling (450–800°C; QUIlF thermometry), and (3) low fO₂ (log fO₂ ΔFMQ −0.3 to −1.5; QUIlF equilibria). In particular, plutons with olivine- and pyroxene-bearing margins and amphibole-dominated central parts record progressive oxidation and hydration upon cooling, shifting from the QUIlF equilibrium toward KUIlB. The post-kinematic granites can be considered post-collisional in regard to compressional events in the CFGC and display many of the characteristics of the anorogenic 1.6 Ga rapakivi granites further south. They were presumably derived from a deep and dry crustal source, like the rapakivi granites.     

库鲁克塔格地区古元古界磁铁石英岩型铁矿的发现及找矿意义

通过塔里木地台北缘找矿研究,在库鲁克塔格地区首次发现磁铁石英岩型铁矿床,铁矿赋存于前寒武系兴地塔格群沙依提组地层中。对该矿床含矿岩系特征、岩矿石化学组成、微量元素、稀土元素组成与矿床地质特征的调查与研究表明,鲁克塔格地区古元古界兴地塔格群沙依提组含铁石英岩主要分布于兴地河—辛格尔一带,含铁建造呈东西向展布。在兴地河-辛格尔地区发现含铁石英岩建造出露长约160km,找矿前景可观。     

The Moreira Gomes deposit of the Cuiú-Cuiú goldfield: Fluid inclusions and stable isotope constraints and implications for the genesis of granite-hosted gold mineralization in the Tapajós Gold Province,Brazil

Moreira Gomes is a recently discovered deposit (21.7 t Au) of the Cuiú-Cuiú goldfield, Tapajós Gold Province, Amazonian Craton. The mineralized zone is about 1200 m long, 30–50 m wide, and at least 400 m in depth. The zone is controlled by a subvertical, east–west-trending structure that is related to a left lateral strike-slip fault system. The host rocks are predominantly tonalites of the Creporizão Intrusive Suite (1997 ± 2 Ma) of uncertain tectonic setting (magmatic arc or post-collision). Hydrothermal alteration and mineralization are predominantly of the fissure-filling type and locally pervasive. Sericitization, chloritization, sulfidation, silicification, carbonatization and epidotization are the observed alteration types. Pyrite is the predominant sulfide mineral and bears inclusions of chalcopyrite, galena, sphalerite and minor hessite and bismuthinite. Gold occurs predominantly as inclusions in pyrite and subordinately in the free-milling state in quartz veins. Ag, Pb and Bi have been detected by semi-quantitative EDS analysis.Three types of fluid inclusions, hosted in quartz veins and veinlets, have been identified. (1) one- and two-phase CO₂ inclusions; (2) two- and three-phase H₂O–CO₂-salt inclusions, and (3) two-phase H₂O-salt inclusions. The CO₂-bearing types are interpreted as the product of phase separation of an immiscible fluid. This fluid presents low to moderate density, low to moderate salinity (1.6–11.8 wt.% NaCl equivalent) and was trapped at 280° to 350 °C. The chemical system of the aqueous inclusions may contain CaCl₂ and/or MgCl₂, salinity varies from zero to 10.1 wt.% NaCl equivalent. Only locally salinities up to 25% have been recorded. This fluid was trapped between 120° and 220 °C and is interpreted as resulting from mixing of a hotter and more saline aqueous fluid (in part derived from phase separation of the H₂O–CO₂ fluid) with a cooler and dilute aqueous fluid.The δ³⁴S values of pyrite (−0.3‰ to 3.9‰) are probably related to magmatic sulfur. The isotopic composition of inclusion fluids and of the fluid in equilibrium with hydrothermal minerals (quartz, chlorite, and calcite) show δ¹⁸O and δD values that range from +0.5 to +9.8‰, and from −49 to −8‰, respectively. Mineral pairs show equilibrium isotopic temperatures that are compatible with the fluid inclusion homogenization temperatures and with textural relationships of the hydrothermal minerals.Isotopic results combined with mineralogical and fluid inclusion data are interpreted to reflect a magmatic-hydrothermal system that evolved in at least three stages. (1) Exsolution of a CO₂-bearing magmatic fluid between 400 °C and 320–350 °C and up to 2.1 kbar (6 km in depth) followed by phase separation and main precipitation of the hydrothermal assemblage composed of chlorite–sericite–pyrite–quartz-gold. (2) Cooling and continuous exsolution of CO₂ produced a CO₂-depleted and slightly more saline aqueous fluid that was trapped mainly at 250°–280 °C. The predominant hydrothermal assemblage of stage 1 continued to form, but epidote is the main phase at this stage. (3) Mixing of the stage 2 aqueous fluid with a cooler and dilute aqueous fluid of meteoric origin, which was responsible for the main carbonatization phase. The mineralizing fluid was neutral to slightly alkaline and relatively reduced. H₂S (and/or HS^-) might have been the main sulfur species in the fluid and Au(HS)^2- was probably the gold transporting complex. Gold deposition occurred as a consequence of a combination of mechanisms, such as phase separation, mixing and fluid-rock interaction.The Moreira Gomes is a granite-hosted gold deposit that is interpreted to be a product of a magmatic-hydrothermal gold system. The age of ore formation (∼1.86 Ga) is consistent with the final stages of evolution of the widespread high-K, calc-alkaline Parauari Intrusive Suite, although the transitional to predominantly alkaline Maloquinha Intrusive Suite cannot be ruled out. Notwithstanding, the deposit does not show the classic features of (oxidized or reduced) intrusion-related gold deposits of Phanerozoic magmatic arcs.     

内蒙古大青山地区古元古代花岗岩:地球化学、锆石SHRIMP定年及其地质意义

内蒙古大青山地区沿固阳-武川断裂带南侧发现一条古元古代花岗岩岩带,由石英闪长岩-闪长岩-角闪二长花岗岩组合构成.本文对其中典型代表厂汉脑包石英闪长岩、常福龙闪长岩和口子村角闪二长花岗岩进行了锆石定年和地球化学研究.地球化学上,石英闪长岩具有埃达克质花岗岩的特点,为低硅埃达克质花岗岩;大部分闪长岩具有赞岐岩的地球化学特征,部分具Closepet花岗岩特征;角闪二长花岗岩则全部具有Closepet花岗岩特征.根据锆石SHRIMP U-Pb定年,它们形成于2416~2435Ma之间,并遭受古元古代晚期构造热事件改造.表明在该区古元古代早期真正意义的板块构造已起作用,从岩浆演化的角度说明前寒武纪构造体制发生重大转变.     

华北克拉通南缘小秦岭地区花岗质浅色脉体锆石U-Pb年龄、稀土元素特征及其地质意义

小秦岭地区发育有大量古元古代晚期的花岗质浅色脉体, 这为理解同时期华北克拉通南缘的深熔作用和构造演化提供了重要的窗口。本文对小秦岭地区三条花岗质浅色脉体中的锆石开展了CL图像、LA-ICP-MS U-Pb年代学及稀土元素的研究。浅色脉体中的锆石具有深熔成因锆石的斑杂状、平面状或弱振荡环带的内部结构及较为自形的外部形态特征, 测得的锆石207Pb/206Pb年龄分别为(1867±13) Ma (MSWD=0.22, n=21)、(1849±17) Ma (MSWD=0.79)和(1828±15) Ma (MSWD=0.33, n=20), 代表了该区深熔作用发生的时间。与上述三组年龄相对应, 其锆石稀土元素配分曲线由具较弱负Eu异常(Eu/Eu*=0.12~0.81)的重稀土相对富集型, 转变为中等负Eu异常(Eu/Eu*=0.29~0.61)的近平坦重稀土型、再到显著负Eu异常(Eu/Eu*=0.15~0.54)的重稀土富集型, 结合锆石Ti温度计获取其结晶温度为574~708 °C, 表明地壳深熔作用的发生可能与高角闪岩相-麻粒岩相的退变质作用有关。结合前人的变质年代学数据可知, 小秦岭地区1.87~1.82 Ga的深熔作用应与~1.95 Ga时东、西部陆块碰撞造山所致增厚地壳的长期抬升和冷却过程有关。     

Precambrian Tectonic Affinity of the Southern Langshan Area, Northeastern Margin of the Alxa Block: Evidence from Zircon U-Pb Dating and Lu-Hf Isotopes

The Alxa Block is the westernmost part of the North China Craton (NCC), and is regarded as one of the basement components of the NCC. Its geological evolution is of great significance for the understanding of the NCC. However, the Precambrian basement of the Alxa Block is still poorly studied. In this study, we present new in situ LA-ICP-MS zircon U-Pb and Lu-Hf isotope data from the Diebusige Metamorphic Complex (DMC) which located in the eastern Alxa Block. Field and petrological studies show that the DMC consists mainly of metamorphic supracrustal rocks and minor metamorphic plutonic rocks and has experienced amphibolite-granulite facies metamorphism. Zircon U-Pb dating results suggested that the amphibolite sample yields a crystallization age of 2636 ± 14 Ma and metamorphic ages of 2517–2454 Ma and 1988–1952 Ma, proving the existence of exposed Archean rocks in the Langshan area and indicating that late Neoarchean to Paleoproterozoic metamorphic events existed in the Alxa Block. Two paragneiss samples show that the magmatic detrital zircons from the DMC yield 207Pb/206Pb ages ranging from 2.48 Ga to 2.10 Ga with two youngest peaks at 2.13 Ga and 2.16 Ga, respectively, and they were also overprinted by metamorphic events at 1.97–1.90 Ga and 1.89–1.79 Ga. Compilation of U-Pb ages of magmatic detrital and metamorphic zircons suggested that the main part of the DMC may have been formed at 2.1–2.0 Ga. Zircon Lu-Hf isotope data show that the source materials of the main part of the DMC were originated from the reworking of ancient Archean crust (3.45–2.78 Ga). The Hf isotope characteristics and the tectonothermal event records exhibit different evolution history with the Khondalite Belt and the Yinshan Block and the other basements of the Alxa Block, indicating that the Langshan was likely an independent terrain before the middle Paleoproterozoic and was subjected to the middle to late Paleoproterozoic tectonothermal events with the Khondalite Belt as a whole.     

Lu–Hf constraints on pre-, syn,and post-collision associations of the Gurupi Belt,Brazil: Insights on the Rhyacian crustal evolution

The Gurupi Belt (together with the São Luís cratonic fragment), in north-northeastern Brazil, has been described in previous studies that used extensive field geology, structural analysis, airborne geophysics, zircon U–Pb dating, and whole-rock Sm–Nd isotope and geochemical data as a polyphase orogenic belt, with the Rhyacian being the main period of crust formation. This was related to a 2240 Ma to 2140 Ma accretionary processes that produced juvenile crust, which has subsequently been reworked during a collisional event at 2100 ± 20 Ma, with little evidence of Archean crust. In this study, we use Lu–Hf isotopic data in zircon from granitoids (including gneiss) of variable magmatic series, and amphibolite to improve the knowledge of this scenario, and investigate additional evidence of recycling of Archean basement. Pre-collisional high Ba-Sr and ferroan granitoids and amphibolite formed in island arc (2180–2145 Ma), show only zircons with suprachondritic εHf values (ca. +1 to +8) indicating the large predominance of juvenile magmas. Only 10% of the data show slightly negative εHf values (0 to ?4), which have been observed in granodiorite-gneiss formed in continental arc (2170–2140 Ma), and in strongly peraluminous collisional granites (2125–2070 Ma), indicating the rework of older Paleoproterozoic to Archean components (Hf_TDM = 2.11–3.69 Ga). A two-component mixing model using both Hf and published Nd isotope data are in line with this interpretation and indicate more than 90% of juvenile material, and less influence of Archean materials. Comparing with other Rhyacian terranes that are interpreted to have been close to Gurupi in a pre-Columbia configuration (ca. 2.0 Ga), our results differ from those of SE-Guiana Shield, which show strong influence of Archean protoliths, and are very similar to those of the central-eastern portion of the Baoulé-Mossi Domain of the West African Craton, which has also been formed largely by juvenile magmas in an accretionary-collisional orogen.