Chemical geodynamics of Western Anatolia

We report major and trace element concentrations and Nd–Sr–Pb isotopic data of 10 post-collisional volcanic domains in Western Anatolia, a seismically active part of the Alpine–Himalayan belt in the Aegean extensional province. Our objective is to provide geochemical constraints for tectono-magmatic processes shaping the late Cenozoic geodynamic evolution of Western Anatolia.

Calc-alkaline volcanic rocks occurring to the north of the Izmir–Ankara–Erzincan suture zone show arc-like trace elements and isotopes and were formed by the melting of the metasomatized Neotethyan mantle-wedge; this process was facilitated by asthenospheric upwelling resulting from slab delamination. Calc-alkaline and alkaline volcanic rocks from within the Izmir–Ankara–Erzincan suture zone also show the imprint of subduction fluids in their major and trace elements, but their isotopic compositions indicate derivation from a metasomatized lithospheric mantle followed by assimilation of ancient crust. Volcanics along the N–S-oriented Kirka–Afyon–Isparta trend were derived from the lithospheric mantle that was metasomatized by fluids from the older subduction of the African plate. Golcuk–Isparta volcanic rocks show an asthenospheric imprint; the latter was a consequence of upwelling following a tear in the subducting African lithosphere. Shoshonitic Kula volcanic rocks show very high trace element concentrations, OIB mantle-like trace elements, and Nd–Sr–Pb isotopic signatures, and were formed by partial melting of the upwelling asthenospheric mantle; this event was synchronous with the Aegean extension and possibly also with slab window formation due to ruptures in the African plate.

Inherent in the above chemical geodynamic models are the high ?_Nd(0) values (+6.4) of the end-member volcanic rocks, implying the presence of an asthenospheric source beneath Western Anatolia that is responsible for the currently observed high heat flow, low Pn wave velocities, high seismicity, and tectonic activity.     

Geochronology and magmatic evolution of the Huautla volcanic field: last stages of the extinct Sierra Madre del Sur igneous province of southern Mexico

The Huautla volcanic field (HVF), in the Sierra Madre del Sur (SMS), is part of an extensive record of Palaeogene magmatism reflecting subduction of the Farallon plate along the western edge of North America. Igneous activity resulting from Farallon subduction is also exposed to the north, in the Sierra Madre Occidental (SMO) and Mesa Central (MC) provinces. We present the results of a stratigraphic and K–Ar, Ar–Ar, and U–Pb geochronological study of the Huautla volcanic successions, in order to refine our knowledge on the petrologic and temporal evolution of the northern SMS and gain insights on magmatic–tectonic contrasts between the SMS and the SMO–MC provinces. The HVF is made up of lava flows and pyroclastic successions that overlie marine Cretaceous sequences and post-orogenic continental deposits of Palaeogene age. In the study area, the main Oligocene succession is pre-dated by the 36.7 million years its caldera west of the Sierra de Huautla. The HVF succession ranges in age from ～33.6 to 28.1 Ma and comprises a lower group of andesitic–dacitic lava flows, an intermediate sequence of ignimbrites and dacitic lavas, and an upper group of andesitic units. The silicic succession comprises a crystal-poor ignimbrite unit (i.e. the Maravillas ignimbrite; 31.4 ± 0.6, 32.0 ± 0.4 Ma; ～260 km³), overlain by a thick succession of dacitic lavas (i.e. the Agua Fría dacite; 30.5 ± 1.9, 31.0 ± 1.1 Ma). Integration of the new stratigraphic and geochronological data with prior information from other explosive centres of the north-central SMS allows us to constrain the temporal evolution of a silicic flare-up episode, indicating that it occurred between 37–32 Ma; it consisted of three major ignimbrite pulses at ～36.5, ～34.5, and ～33–32 Ma and probably resulted from a progressive, mantle flux-driven thermomechanical maturation of the continental crust, as suggested in the HVF by the transition from andesitic to voluminous siliceous volcanism. The information now available for the north-central sector of the SMS also allows recognition of differences between the temporal and spatial evolution of magmatism in this region, and of that documented in the southern SMO and MC provinces, suggesting that such contrasts are probably related to local differences in configuration of the subduction system. At ～28 Ma, the MC and southern SMO provinces experienced a trenchward migration of volcanism, associated with slab rollback; on the other hand, the broad, more stable distribution of Oligocene magmatism in the central and north oceanic plate was subducting at a low angle.     

Petrogenesis of mid-Carboniferous volcanics and granitic intrusions from western Junggar Basin boreholes: geodynamic implications for the Central Asian Orogenic Belt in Northwest China

The western Junggar Basin is located on the southeastern margin of the West Junggar terrane, Northwest China. Its sedimentary fill, magma petrogenesis, tectonic setting, and formation ages are important for understanding the Carboniferous tectonic evolution and continental growth of the Junggar terrane and the Central Asian Orogenic Belt. This paper documents a set of new zircon secondary ion mass spectrometry U–Pb geochronological and Hf isotopic data and whole-rock elemental and Sr–Nd isotopic analytical results for the Carboniferous strata and associated intrusions obtained from boreholes in the western Junggar Basin. The Carboniferous strata comprise basaltic andesite, andesite, and dacite with minor pyroclastic rocks, intruded by granitic intrusions with zircon secondary ion mass spectrometry U–Pb ages of 327–324 Ma. The volcanic rocks are calc-alkaline and show low high ε_Nd(t) values (5.3–5.6) and initial ⁸⁷Sr/⁸⁶Sr (0.703561–0.703931), strong enrichment in LREEs, and some LILEs and depletion in Nb, Ta, and Ti. Furthermore, they also display high (La/Sm)_N (1.36–1.63), Zr/Nb, and La/Yb, variable Ba/La and Ba/Th and constant Th/Yb ratios. These geochemical data, together with low Sm/Yb (1.18–1.38) and La/Sm (2.11–2.53) ratios, suggest that these volcanic rocks were derived from a 5–8% partial melting of a mainly spinel Iherzolite-depleted mantle metasomatized by slab-derived fluids and melts of some sediments in an island-arc setting. In contrast, the granitic intrusions represent typical adakite geochemical features of high Sr and low Y and Yb contents, with no significant Eu anomalies, high Mg#, and depleted ε_Nd(t) (5.6–6.4) and ε_Hf(t) (13.7–16.2) isotopic compositions, suggesting their derivation from partial melting of hot subducted oceanic crust. In combination with the previous work, the West Junggar terrane and adjacent western Junggar Basin are interpreted as a Mariana-type arc system driven by northwestward subduction of the Junggar Ocean, possibly with a tectonic transition from normal to ridge subduction commencing ca. at 331–327 Ma.     

Oligocene–Miocene geodynamic evolution of the central part of Urumieh-Dokhtar Arc of Iran

Basic volcanic rocks from Tafresh, west Kashan, and west Nain volcanic successions in the central part of Urumieh-Dokhtar Magmatic Assemblage (UDMA) of Iran yield K–Ar ages ranging from 26.8 to 18.2 Ma. These ages indicate significant Late Oligocene–Early Miocene basic volcanism in the UDMA. These ages, combined with K–Ar ages of 26.0 and 14.1 Ma, respectively, for associated low-silica and high-silica adakites, help constrain reconstructions of the UDMA geodynamic evolution. Late Oligocene–Early Miocene slab roll-back associated with an asthenospheric mantle influx are suggested as the major processes responsible for concurrent volcanism showing Nb–Ta-depleted, Nb–Ta-enriched and low-silica adakite signatures. Slab roll-back, the likely consequence of a decrease in subduction velocity, led to partial melting of the subducted slab and produced Early–Middle Miocene high-silica (dacitic) adakites. Oligocene to Miocene volcanic rocks do not conform to the Oligocene continental collisional model for the UDMA, rather they suggest a decrease in the subduction rate that prompted the asthenospheric mantle influx.     

江西省兴源冲地区深部找矿潜力评价

江西省兴源冲地区处于九岭南缘逆冲推覆构造系中,NEE向的慈化—宜丰板缘深断裂带及大型推（滑）覆构造西段向南呈弧型转弯并与湘赣边界的NNE向走滑推覆冲断带复合,形成了对铜多金属矿成矿有利的异常构造、岩浆岩、矿源层及赋矿层、矿化异常等控矿因素耦合区。该区具有明显的地、物、化、遥找矿信息：区域1∶20万和1∶5万水系沉积物测量显示Cu,Pb,Zn高异常区;1∶2.5万和1∶1万土壤地化测量已圈定出Cu,Pb,Zn,Au,Ag元素异常多处,综合异常4处;1∶1万激电异常明显;少量探槽、钻孔控制求得333＋3341资源量已具小型规模。根据成矿地质条件及物、化异常分析,该区深部找矿潜力较大,有望扩大为中型以上规模铜矿床。     

福建柘荣马坑银矿地质特征及找矿方向

福建省柘荣县马坑银矿产于上侏罗统-下白垩统陆相酸性火山岩中,矿体严格受断裂（F1,F2）及其派生裂隙的控制。矿床发育绢云母化、硅化、黄铁矿化、黄铁绢英岩化、青磐岩化等中低温热液蚀变,矿体地表呈密集细脉状,深部为简单板状、透镜状。矿石矿物以自然银为主,次为辉银矿,含少量方铅矿、闪锌矿等。矿石工业类型为硫化矿,矿床成因为受断裂构造控制的中低温浅成火山热液充填交代型银矿床。     

中条裂谷的幔柱构造与成矿

中条山地区的地球物理场具有“三低一高”(低波速、低密度、低黏度及高地热异常)特点,变质核杂岩中赋存有科马堤岩并具“双层”结构,火成岩呈“双峰”状态,变质期的古地温为522～608℃,压力0.3～0.73GPa,地热梯度23℃/km,表明中条山地区为新太古代-元古宙的大陆裂谷—地幔热柱构造.其成矿作用存在着“四代同堂”的成矿模式,成矿期与岩浆侵入期的时差甚小;成矿空间上,大多数矿床沿着裂谷内两组断裂呈X型“对称式”展布,有益元素矿物组合及成矿温度自中心向周围呈有规律降低趋势.由于在新太古代以前,富铜的扬子板块俯冲至华北板块之下,促使中条裂谷的地壳加厚,形成丰富的成矿物质场,因为地幔热柱脉动式隆升,热液在运移过程中不断萃取地壳的铜质,故成为区域内独特的铜矿集中区.     

赣北石门寺矿区钨多金属矿床成矿地质条件

赣北石门寺矿区位于下扬子成矿省江南地块中生代铜钼金银铅锌成矿带中.根据区域地质背景和矿床地质特征,详细分析了矿区围岩、母岩和控矿断裂等成矿地质条件.矿区钨多金属矿床的围岩为晋宁晚期黑云母花岗闪长岩,判断为新元古代在不成熟陆壳基础上发育而成的火山弧同碰撞过程中形成的S型花岗岩;燕山中期似斑状黑云母花岗岩、细粒黑云母花岗岩、花岗斑岩为成矿母岩,属硅、铝过饱和钙碱性岩石,为九岭岩基在陆内碰撞挤压环境下熔融、同源演化而成的S型花岗岩;石门寺断裂与仙果山—大湖塘－狮尾洞基底断裂的交叉部位控制着矿区燕山中期含矿花岗岩的侵位和钨多金属矿床的分布,为矿区的控矿断裂.     

湘西南铜山岭地区钨多金属矿床地质特征及成矿机制探讨 ——以祥霖铺矿床为例

祥霖铺夕卡岩型钨多金属矿床位于南岭纬向构造带的中段,矿床产于NE向、NW向和EW向断裂的交汇处,矿化集中赋存于背斜的倾伏端附近.通过对矿床地质特征、地球物理、地球化学特征等方面的研究,认为燕山期构造运动所导致的岩浆活动为矿床的形成提供了大量热源,区内的断裂构造为成矿提供了良好的构造条件,中泥盆统棋梓桥组在成矿过程中提供了大量的物质来源;在夕卡岩化过程中形成了不同的矿物组合,成矿过程可以分为2个成矿期、4个成矿阶段;预测在矿区的西部有望找到隐伏矿体.     

辽宁岫岩五道沟铁矿床地质特征

辽宁省岫岩县五道沟铁矿产于古元古界辽河群里尔峪组变质岩中,层位控矿的特点明显;铁矿体的围岩为黑云斜长变粒岩,矿体与围岩界线清晰;矿体的围岩蚀变不甚强烈,晚期构造和闪长玢岩脉穿切铁矿体,但对铁矿体无明显的破坏。该矿床与区域上杨林式铁矿可以对比,矿床成因属于火山沉积-变质铁矿,成矿时代为古元古代。