Zircon and molybdenite geochronology and geochemistry of the Kalmakyr porphyry Cu–Au deposit,Almalyk district,Uzbekistan: Implications for mineralization processes

Copper, gold and molybdenum mineralization of the Kalmakyr porphyry deposit in Uzbek Tien Shan occurs as stockworks, veinlets and disseminations in the phyllic and K-silicate alteration zones developed predominantly in a middle to late Carboniferous intrusive complex composed of monzonite and granodiorite porphyry. Zircon U–Pb dating yielded an age of 327.2 ± 5.6 Ma for the ore-hosting monzonite and an age of 313.6 ± 2.8 Ma for the ore-bearing granodiorite porphyry. Re–Os dating of seven molybdenite samples from stockwork and veinlet ores yielded model ages from 313.2 to 306.3 Ma, with two well-constrained isochron ages of 307.6 ± 2.5 Ma (five stockwork ores) and 309.1 ± 2.2 Ma (five stockwork ores and two veinlet ores), respectively. These results indicate that Cu–Au mineralization post-dated the emplacement of the monzonite, started right after the emplacement of the granodiorite porphyry, and lasted for ca. 7 Ma afterward. The geochronological and geochemical data suggest that the Kalmakyr deposit was formed in a late Carboniferous mature magmatic arc setting, probably related to the latest subduction process of the Turkestan Ocean beneath the Middle Tien Shan. The ε_Hf(t) values of zircon grains from the monzonite vary from +11 to +1.7, with an average of +5.1, and those of zircon grains from the granodiorite porphyry range from +5.7 to −1.8, with an average of +2.4. These data indicate that the magma of both monzonite and granodiorite porphyry was derived from partial melting of a thickened lower crust with input of mantle components and variable crustal contamination, and that there was more mantle contribution to the formation of the monzonite than the granodiorite porphyry. The high rhenium concentrations of molybdenite (98–899 ppm) also indicate major mantle contribution of rhenium and by inference ore metals. The relatively high Eu_N/Eu_N¹ values (average 0.68), Ce⁴⁺/Ce³ values (average 890) and Ce/Nd values (average 36.8) for zircon grains from the granodiorite porphyry than those from the monzonite (average Eu_N/Eu_N¹ = 0.33, average Ce⁴⁺/Ce³ = 624, average Ce/Nd = 3.9) suggest that the magma for the syn-mineralization granodiorite porphyry has higher oxygen fugacity than that for the pre-mineralization monzonite. Based on these data, it is proposed that while the monzonite was emplaced, the oxygen fugacity and volatile contents in the magma were relatively low, and ore metals might disperse in the intrusive rock, whereas when the granodiorite porphyry was emplaced, the oxygen fugacity and volatile contents in the magma were increased, favoring copper and gold enrichment in the magmatic fluids. The Kalmakyr deposit formed from a long-lived magmatic-hydrothermal system connected with fertile magmatic sources in relation to the subduction of the Turkestan Ocean beneath the Middle Tien Shan.     

青藏铁路风火山段晚第四纪断裂活动分析

地表地质调查发现,第四纪期间在风火山逆冲-褶皱构造带以发生近东西向的伸展变形为特征。在该构造带中形成切割早期近东西向挤压变形构造带、指示近东西向伸展变形、整体沿北60°东向展布的二道沟断陷盆地。断裂活动的地质、地貌证据表明,控制该盆地晚第四纪断陷的主边界断裂位于其北缘,是一条断续延伸达24 km左右、可能兼具左旋走滑性质的正断层。根据该区晚第四纪沉积物的分布和时代,并对断裂所错动的晚第四纪地质-地貌体进行初步的年代学分析,可以初步断定该断裂的晚第四纪垂直活动速率应该介于0.2~0.4 mm/a之间。     

Multiple generations of pseudotachylyte in the brittle to ductile regimes, Qinling–Dabie Shan ultrahigh-pressure metamorphic complex, central China

Abstract Pseudotachylytes are present along the Dahezhen shear zone in the Qinling–Dabie Shan collisional orogenic belt, central China. Two types of pseudotachylyte vein are documented in the shear zone: cataclasite‐related pseudotachylyte (C‐Pt) and mylonite‐related pseudotachylyte (M‐Pt). M‐Pt is associated with mylonite‐development and is overprinted by C‐Pt. All of the quartz and most of the feldspar porphyroclasts within the M‐Pt are plastically deformed, but not in the C‐Pt. Dynamically recrystallized fine‐grained quartz and feldspar bands are oriented subparallel to the mylonite and M‐Pt foliation, and partially surround the porphyroclasts. Our results suggest that the M‐Pt formed cyclically in the ductile region at estimated conditions of 400–650°C and 400–800 MPa due to propagation of seismic fracturing associated with the thrusting‐related rapid exhumation of the ultrahigh‐pressure metamorphic complex in the brittle regime down to a greater depth than the base of the seismogenic zone. The M‐Pt and mylonite formed in the Dahezhen shear zone at estimated conditions of 400–650°C and 400–800 MPa. The coexistence of C‐Pt and M‐Pt in the same shear zone suggests that repeated seismic slips occurred in both the brittle and ductile portions of the crust during the thrusting‐related rapid exhumation of the ultrahigh‐pressure metamorphic complex.     

Metamorphic zircon from Xindian eclogite,Dabie Terrain: U-Pb age and oxygen isotope composition

~~Metamorphic zircon from Xindian eclogite,Dabie Terrain: U-Pb age and oxygen isotope composition@E. Deloule$CRPG-CNRS Nancy,54501,France1. Vavra, G, Gebauer. D., Schmid. R. et al., Multiple zircon growth and recrystallization during polyphase Late Carboniferous to Tri-assic metamorphism in granulites of the Ivrea Zone (Southern Alps): an ion microprobe (SHRIMP) study, Contrib. Mineral Petrol., 1996, 122:337-358 2. Vavra, G, Schmid, R., Gebauer, D., Internal morphology, ha…     

龙门山断裂带北段深部结构与反射地震特征

2008年5月12日汶川M_W7.9特大地震发生在龙门山断裂带,龙门山断裂带深部结构的复杂性制约了地震的破裂过程.通过对研究区区域地质、汶川地震前后采集的地震反射剖面等研究,在对龙门山北段汶川地震断裂带的深部结构和反射地震特征进行了分析的基础上,探讨了它对地表破裂过程的制约.研究结果表明,在地震剖面上,断裂带表现为能量破碎、联系性差;频率剖面上显示整体剖面频率在5~45 Hz,断裂带呈现频率低（15~26 Hz）等特征.龙门山北段映秀—北川断裂在10 km以上是一条倾向北西的高角度走滑兼逆冲性质的断裂,倾角50°~70°.它分割了西侧的轿子顶杂岩和东侧的唐王寨推覆体,错断了早期形成的逆冲岩片,从南到北总位移量由大变小.它高角度的几何形态约束了断裂以走滑为主兼逆冲分量的运动性质,降低了地表滑移量,影响了地震破裂过程以及余震沿断裂带两侧分布的特性.

     

合黎山地区地质构造特征及成矿预测

本文依据最新的1:5万高精度航磁资料对甘肃合黎山地区的基底构造性质、断裂构造及岩浆岩分布特征进行了初步探讨,并在此基础上预测了区内金及多金属矿产的找矿远景。     

Quaternary folding of the eastern Tian Shan, northwest China

The Tian Shan, east–west trending more than 2000 km, is one of most active intracontinental mountain building belts that resulted from India–Eurasia collision during Cenozoic. In this study, Quaternary folding related to intracontinental mountain building of the Tian Shan orogenic belt is documented based on geologic interpretation and analyses of the satellite remote sensing images [Landsat Thematic Mapper (TM)/Enhanced Thematic Mapper (ETM) and India Remote Sensing (IRS) Pan] combined with field geologic and geomorphic observations and seismic reflection profiles. Analyses of spatial–temporal features of Quaternary folded structure indicate that the early Quaternary folds are widely distributed in both piedmont and intermontane basins, whereas the late Quaternary active folds are mainly concentrated on the northern range-fronts. Field observations indicate that Quaternary folds are mainly characterized by fault-related folding. The formation and migration of Quaternary folding are likely related to decollement surfaces beneath the fold-and-fault zone as revealed by seismic reflection profiles. Moreover, analysis of growth strata indicates that the Quaternary folding began in late stage of early Pleistocene (2.1–1.2 Ma). Finally, tectonic evolution model of the Quaternary deformation in the Tian Shan is presented. This model shows that the Quaternary folding and faulting gradually migrate toward the range-fronts due to the continuous compression related to India–Eurasia collision during Quaternary time. As a result, the high topographic relief of the Tian Shan was formed.     

Morphological Comparison between Chinese Ayu and Japanese Ayu and Establishment of Plecoglossus altivelis chinensis Wu ＆ Shan subsp, nov.

The countable characters of Qingdao, Zhejiang, Liaoning and Japanese Ayu are compared. There are no substantial different characters among the former three Ayu, so the comprehensive values of them are taken as representative of Chinese Ayu, which is in turn compared with Japanese Ayu (Plecoglossus altivelis altivelis and Plecoglossus altivelis ryukyuensis). By the coefficient of difference test, 3 and 4 characters are found to be beyond the subspecies level between Chinese Ayu and nominate subspecies (Plecoglossus altivelis altivelis) and between Chinese Ayu (Plecoglossus altivelis chinensis) and Ryukyus subspecies (Plecoglossus altivelis ryukyuensis), respectively, which shows that they belong to different subspecies. The China mainland Ayu is a new subspecies, Plecoglossus altivelis chinensis Wu & Shan, the establishment of which and its forming causes are discussed.     

Geochemical constraints of the eclogite and granulite facies metamorphism as recognized in the Raobazhai complex from North Dabie Shan,China

A combined study of major and trace elements, fluid inclusions and oxygen isotopes has been carried out on garnet pyroxenite from the Raobazhai complex in the North Dabie Terrane (NDT). Well‐preserved compositional zoning with Na decreasing and Ca and Mg increasing from the core to rim of pyroxene in the garnet pyroxenite indicates eclogite facies metamorphism at the peak metamorphic stage and subsequent granulite facies metamorphism during uplift. A P–T path with substantial heating (from c. 750 to 900 °C) after the maximum pressure reveals a different uplift history compared with most other eclogites in the South Dabie Terrane (SDT). Fluid inclusion data can be correlated with the metamorphic grade: the fluid regime during the peak metamorphism (eclogite facies) was dominated by N₂‐bearing NaCl‐rich solutions, whereas it changed into CO₂‐dominated fluids during the granulite facies retrograde metamorphism. At a late retrograde metamorphic stage, probably after amphibolite facies metamorphism, some external low‐salinity fluids were involved. In situ UV‐laser oxygen isotope analysis was undertaken on a 7 mm garnet, and impure pyroxene, amphibole and plagioclase. The nearly homogeneous oxygen isotopic composition (δ¹⁸O_VSMOW = c. 6.7‰) in the garnet porphyroblast indicates closed fluid system conditions during garnet growth. However, isotopic fractionations between retrograde phases (amphibole and plagioclase) and garnet show an oxygen isotopic disequilibrium, indicating retrograde fluid–rock interactions. Unusual MORB‐like rare earth element (REE) patterns for whole rock of the garnet pyroxenite contrast with most ultra‐high‐pressure (UHP) eclogites in the Dabie‐Sulu area. However, the age‐corrected initial ε_Nd(t) is ? 2.9, which indicates that the protolith of the garnet pyroxenite was derived from an enriched mantle rather than from a MORB source. Combined with the present data of oxygen isotopic compositions and the characteristic N₂ content in the fluid inclusions, we suggest that the protolith of the garnet pyroxenite from Raobazhai formed in an enriched mantle fragment, which has been exposed to the surface prior to the Triassic metamorphism.     

活动构造对高原边界侵蚀速率空间分布的控制作用——以龙门山地区为例

利用穿过龙门山地区的河流水文与泥沙资料估算该地区现代侵蚀速率,进而研究探讨其空间分布的不均匀性。年代尺度的现代侵蚀速率空间分布表明,高侵蚀区分布于控制高原边界的龙门山活动断裂带和岷山活动断裂带附近且位于逆冲断裂上盘宽约50km范围内,其值达0.5~0.8mm/a。在地形上,该高侵蚀速率区对应于陡峭高原边界的地形陡坡带。自龙门山断裂带向高原内部,侵蚀速率快速降低,在距离断裂带北西200km的地区侵蚀速率小于0.05mm/a。河流输沙量估算的现代侵蚀速率值与河沙宇宙成因核素估算的千年时间尺度的流域平均速率和低温热年代学方法估算的百万年时间尺度的单点平均侵蚀速率基本一致。季风降雨在龙门山地区也存在自东南向北西递降的趋势,但年降水量自山前的1000mm到高原内部的600mm 的递减梯度较小,不足以解释侵蚀速率的一个数量级的减小。气候对侵蚀作用的影响更多地表现在时间域随季风降雨强度的年及季节性波动而震荡,最突出的表现为80% ~90%侵蚀产物在6~9月被季风降水输移出去。逆冲断裂上盘宽约50km的高侵蚀区以及与地形陡坡带的对应性,体现了断裂上盘物质在从较平的断坪向较陡的断坡区运移过程中抬升增加的侵蚀卸载均衡。0.5~0.8mm/a 的抬升剥蚀率与该地区已知的1.4~3.0mm/a的水平缩短速率匹配,说明龙门山高原边界地形处于通量平衡态,且上地壳缩短与龙门山边界的陡降地形是匹配的。