南岭成矿带大湾铀矿田铀成矿环境与找矿前景

湖南省大湾铀矿田位于南岭成矿带九嶷山杂岩体中段,有利的成矿环境使该地区成为华南重要的产铀区及稀有金属和有色金属矿集区。岩体外接触带寒武系为矿田的主要铀源层之一和成矿有利部位,已探明的铀矿床均产于九嶷山杂岩体中部金鸡岭岩体外接触带。九嶷山杂岩体分别由东、西两个完全独立的岩浆演化中心演化而来,西部岩浆演化中心的金鸡岭岩体与铀成矿作用关系密切,形成了岩浆演化中心、热流体活动中心与矿化中心三位一体的基本构造格局。深大断裂带为成矿流体循环运移提供了通道和热动力来源。铀成矿作用主要发生在晚白垩世早期地应力转向期,矿化与NW向伸展断裂及其相伴的深源热流体有关。岩体接触带为铀沉淀富集的有利地球化学障。重力、航磁及航空γ能谱异常及放射性水化学异常是找铀的重要判据之一。在区域铀成矿环境分析基础上,对该区找矿前景进行了展望。     

华北地块北缘晚古生代-中生代花岗岩体侵位深度及其构造意义

运用斜长石-角闪石温压计对华北地块北缘内蒙古隆起及燕山褶断带内不同时期花岗质侵入岩的结晶压力及侵位深度进行了估算。结果表明,晚古生代—早中生代期间,在内蒙古隆起及燕山褶断带之间,存在有强烈的差异性隆升及剥露过程,但这种差异性隆升及剥露在早侏罗世以来的表现则不明显。晚古生代—早中生代差异性隆升及剥露可能是导致内蒙古隆起上大量基底岩石出露、中—新元古代及古生代沉积盖层缺失及燕山褶断带中—新元古代及古生代沉积盖层大量保留的主要原因。内蒙古隆起强烈的隆升及剥露过程发生在晚石炭世—早侏罗世期间,其东部的剥露幅度比中东部明显偏小。晚古生代-早中生代期间内蒙古隆起的强烈剥露及其与燕山褶断带之间的差异性隆升可能与古亚洲洋板块向华北地块的俯冲、消减、碰撞及华北北缘区域性断裂(如平泉-古北口-赤城-尚义断裂、赤峰-围场-多伦断裂)的活动有关。燕山褶断带的强烈隆升与剥露发生则在晚侏罗世—早白垩世之后。晚体罗世—早白垩世以来,华北地块北缘南北两侧均有一次明显的剥露过程,这一剥露可能与本区及中国东部地壳强烈伸展有关。     

吉林和龙地区勇新二长花岗岩的成因及形成构造环境探讨

通过对吉林和龙地区勇新岩体岩石学、岩石化学、岩石地球化学及锆石U—Pb年代学等方面的研究,探讨其形成时代、岩石成因、形成构造环境等问题。勇新岩体岩石类型为二长花岗岩,与围岩新元古代长仁岩组大理岩接触带上,见有石英、钾长石与方解石共生现象;岩体中见有辉长岩脉;镜下观察,长英质矿物重结晶现象发育,并见有斜长石残斑。岩石化学、地球化学分析结果显示,二长花岗岩高硅富铝贫镁,属于过铝质、高钾钙碱性系列,成因类型为S (I)型花岗岩;δEu值0. 09～0. 77,平均值0. 47,显示为较强烈的Eu负异常,La_N/Yb_N值12. 61～25. 77,平均值22. 57,轻重稀土分馏明显;富集大离子亲石元素Rb、K和Th等活泼的不相容元素,明显亏损Nb、Ta、Zr、P、Ti等高场强元素。LA—ICP—MS锆石U—Pb定年结果表明勇新岩体形成于中三叠世,其年龄分别为236. 9±1. 1 Ma和236. 5±1. 1 Ma。结合区域资料,认为勇新岩体形成于中下地壳的原地重熔,与古亚洲洋闭合后的岩石圈伸展密切相关。     

“花岗岩造岩矿物~(40)Ar-~(39)Ar体系的封闭温度≤500℃”质疑——基于角闪石-黑云母~(40)Ar-~(39)Ar年龄与锆石U-Pb年龄对比的证据及Dodson's公式存在问题剖析

通过国内外花岗岩体的412对角闪石-黑云母40Ar-39Ar年龄(tAr)与锆石U-Pb年龄(tZr)之间差值ΔtZr-Ar(tZr-tAr)进行的频数统计分析表明:ΔtZr-Ar呈对称正态分布(偏度系数CSK==-0.276,峰度系数CKU=16.52);ΔtZr-Ar既呈正值又有负值,其众数值为0.70Ma,均值为1.15Ma。采用最小二乘法计算,花岗岩体锆石U-Pb年龄与花岗岩角闪石-黑云母40Ar-39Ar年龄拟合出相关系数很高(R=0.996),回归系数接近l的线性回归方程(tAr=1.00453×tZr-1.932)。这些统计特征表明花岗岩角闪石-黑云母40Ar-39Ar定年测定结果与锆石U-Pb定年测定结果在允许的误差范围内是一致的,不存在花岗岩锆石U-Pb年龄角闪石40Ar-39Ar年龄黑云母40Ar-39Ar年龄的规律。结合Dodson's矿物封闭温度计算公式中存在问题的剖析,本文得出结论,按Dodson's公式计算得出的花岗岩矿物40Ar-39Ar封闭温度不能代表从花岗岩浆晶出造岩矿物40Ar-39Ar体系的封闭温度。     

桂北宝坛锡多金属成矿区平英岩体晚期侵入岩的年代学、地球化学特征及其地质意义

桂北宝坛锡多金属成矿区内岩浆活动频繁,其中与锡多金属成矿作用有关的平英岩体由早晚两期侵入岩组成,且晚期侵入岩中常含有较多的电英岩包体。岩石学、锆石U-Pb年代学和地球化学研究表明：平英岩体晚期侵入岩主要岩性是（中）细粒斑状黑云母（二长-）碱长花岗岩;晚期侵入岩的形成年龄为（769.2±2.5）Ma,而电英岩包体的形成年龄为（795.1±3.1）Ma;平英岩体总体以富含大离子亲石元素（LILE,包括Rb、Th和U等）、贫高场强元素（HFSE,包括Nb和Ti等）为特征,其早期侵入岩属于高钾钙碱性强过铝质S型花岗岩,具有明显的Th-U元素"U"型峰值以及Ti元素"V"型谷值,晚期侵入岩属于钾玄岩系列岩石,具有U元素"V"型尖峰值以及La-Nd和Eu-Ti元素"U"型谷值,二者在地球化学特征上存在较大的差异。结合岩浆源区性质的研究结果,早期侵入岩属于造山后花岗岩类,可能是源自泥质岩类部分熔融形成的岩浆,再经历高程度分离结晶作用形成的产物;而晚期侵入岩属于非造山花岗岩类,可能是源自泥质岩和砂屑岩类部分熔融形成的岩浆与幔源岩浆发生了不同程度的混合形成的产物。结合电英岩包体和寨滚锡多金属矿床中岩浆热液成矿阶段形成的电英岩脉在成因上存在亲缘关系,且其形成年龄与田棚岩体的形成年龄基本一致的特征,推测电英岩包体应为晚期侵入岩侵位过程中捕获早期侵入岩（田棚岩体）侵位过程中形成的岩浆热液型电英岩脉而形成的包体,其形成年龄可能代表了本区电英岩成矿阶段的成矿年龄。     

Remnants of a Neoproterozoic Collisional Orogenic Belt in the Core of the Phanerozoic Qinling Orogenic Belt (China)

NW-SE trending paleotectonics are recognized from the WNW-ESE trending Qinling complex in the core of the Qinling belt. They consist of trending stratum, boundary shear zones, deformed linear plutons and ophiolite. The field relationships and ages of the deformation, magmatism and metamorphism suggest that the paleotectonics formed in Neoproterozoic. The ages of deformed and undeformed plutons approximately constrain the peak deformation during 958-889 Ma. Both the two trending tectonics constitute an orogen-scale tectonic replacement by S2 (Phanerozoic WNW-ESE tectonics) of S1 (NNW-SSE trending Neoproterozoic), similar to structural replacement in outcrop. The strong contractional deformation, (high-pressure) metamorphism, and granitic magmatism that evolved from syn-collisional (S-type), to post-collisional (I-type) and to post-collisional (A-type), as well as regional extension at 885-700 Ma in Qinling, display a cycle of a collisional orogeny. Thus, the NW-SE trending paleotectonics could be remnants of a collisional orogenic belt. The belt originally could trend NNW-SSE, if reworking and modification of Paleozoic and/or Mesozoic NNE-SSW contractional deformation are eliminated. Accordingly, assemblage and breakup of some continental blocks occurred in Neoproterozoic in Qinling, which seems to correspond to assemblage and breakup of Rodinia supercontinent in the world. This study provides new insight into the evolution of the Qinling orogenic belt and the reconstruction of Proterozoic supercontinent of China, and also presents a case study of recognition of an old orogen from a young orogen.     

内蒙古大青山地区哈拉合少岩体特征及成因

通过对大青山哈拉合少岩体基本地质特征研究,包括岩石矿物学特征、岩石化学和地球化学特征、同位素年代分析等,对岩体形成时代和形成构造环境有了一些认识:锆石U-Pb等时线测年分析结果表明,206Pb/238U表面年龄计权重平均值为261.1 Ma±0.5 Ma,代表哈拉合少岩体侵位时代;岩石化学分析结果和岩石学特征表明,该岩体为I型与S型混合成因花岗岩,主要由壳源物质重熔形成。根据地球化学分析结果,结合形成的构造背景,认为该岩体是在西伯利亚板块向南俯冲与华北板块北缘碰撞晚期的应力松驰阶段形成的,属造山期同碰撞花岗岩。     

Origin of Early Cretaceous Guandian adakitic pluton in central eastern China: partial melting of delaminated lower continental crust triggered by ridge subduction

ABSTRACT

Early Cretaceous adakite or adakitic plutons are widely distributed in central eastern China, e.g. lower Yangtze river belt (LYRB), the south Tan–Lu fault (STLF), and the Dabie orogen. Their genesis, however, remains controversial. In this contribution, we present detailed geochemical and geochronological study on the Guandian pluton in central Anhui Province, eastern China, which has been formerly regarded as a part of the north belt in the LYRB and lately classified in the STLF. Namely, it is located near the boundary between ridge subduction related slab melting and partial melting of lower continental crust (LCC). The Guandian pluton consists of quartz monzonite and is metaluminous and high-K calc-alkaline according to the chemical composition. The samples show high SiO₂ (59.15–62.32%), Al₂O₃ (14.51–15.39%), Sr (892–1184 ppm), Sr/Y (56.74–86.32), and low Y (12.65–18.05 ppm), similar to typical geochemical features of adakite. The Guandian adakitic rocks also exhibit high K₂O (2.88–3.86%), MgO (3.89–5.24%), and Mg# (55–60), negative anomalies of high field strength elements (e.g. Nb, Ta, and Ti), and positive anomalies of Ba, Pb, and Sr. LA-ICP-MS zircon U–Pb dating yielded a weighted average age of 129.2 ± 0.7 Ma. Calculations of zircon Ce⁴⁺/Ce³⁺ (6.97–145) and (Eu/Eu*)_N (0.23–0.42) on the basis of in situ zircon trace element analysis indicate that the magma had a lower oxygen fugacity relative to the ore-bearing adakites in the LYRB and Dexing, which is consistent with the fact of ore-barren in the research area. In combination with previous research, we propose that Guandian adakitic pluton was formed by partial melting of delaminated LCC triggered by Early Cretaceous ridge subduction of the Pacific and Izanagi plates. During ridge subduction, physical erosion destructed the thickened LCC and resulted in delamination, while thermal erosion facilitated partial melting of the delaminated LCC.     

内蒙古浩尧尔忽洞金矿区岩体地球化学特征及成矿意义

内蒙古浩尧尔忽洞金矿位于华北地台北缘西段中元古代白云鄂博台缘凹陷带西部,矿区内有大面积花岗质岩体和岩脉出露,其岩性主要包括石英二长闪长岩、二长花岗岩和碱长花岗岩。通过岩相学特征和岩石地球化学特征分析,结合区域上同类型岩体的侵入时代,认为浩尧尔忽洞岩体形成于华北板块与西伯利亚板块同碰撞-后碰撞环境。其中二长花岗岩形成于同碰撞环境,该环境下由于板片持续俯冲,引起俯冲板片及地幔楔发生熔融,其上侵带来的热量致使下地壳物质部分熔融而形成该类型岩石的母岩浆;石英二长闪长岩形成于碰撞后隆起环境,是加厚下地壳熔融的产物;碱长花岗岩属于晚造山期A型花岗岩系列,该阶段地幔玄武质岩浆底侵,导致下地壳物质熔融,部分与其发生混染。结合浩尧尔忽洞金矿的成矿年龄、成矿流体特征及赋矿岩石有机地球化学特征,认为岩体的侵位提供了矿床形成必不可少的热量及部分成矿流体来源。     

柴达木盆地北缘全吉地块东端变基性岩体LA-ICP-MS锆石U-Pb年龄：中元古代陆块裂解的证据

全吉地块东端古元古代二长花岗片麻岩中普遍发育变质的基性岩体,它们的形成时代、变质作用时代和对地块基底构造演化的指示意义尚不清楚。应用LA-ICP-MS技术测定其中的一个变基性岩体的锆石U-Pb年龄,获得的10个测点数据显示,锆石受到热事件的强烈改造而发生严重铅丢失,但拟合的不一致线仍给出相关性较好的上交点年龄1712Ma±47Ma和下交点年龄472Ma±29Ma,MSWD=1.4。这表明,变质基性岩的原岩形成于中元古代早期,是全吉地块在古元古代末期汇入Columbia超大陆之后于中元古代早期初始裂解而侵位的基性岩体。这些基性岩体在早古生代卷入了柴北缘碰撞带而发生角闪岩相变质作用,并受到构造肢解。