Genesis of Mafic Microgranular Enclaves in the Shaocunwu Granodiorite,Southern China,and their Implications: Evidence from Zircon and Whole-rock Geochemistry

Magmatic microgranular enclaves (MMEs) are widely developed in the Shaocunwu granodiorite at the northeast margin of the eastern Jiangnan orogenic belt.Field geology showed that the MMEs occur as irregular ellipsoids near the edge of the intrusion,and consist of diorite,dominantly composed of amphibole,biotite,and plagioclase grains,with minor acicular apatite.Zircon U-Pb dating showed the ages of the host granodiorites and MMEs are 145.9±1.1 Ma and 145.6±2.5 Ma,respectively,indicating both originated during coeval late Jurassic magmatism.Whole-rock geochemical results show that the host granodiorite and MMEs have similar rare earth and trace element partition curves in spider grams,and similar ~(87)Sr/~(86)Sr,and ~(147)Nd/~(144)Nd isotope ratios,and their zircon ~(177)Hf/~(176)Hf isotopic ratios are similar.Geochemical studies indicate that both the host granodiorite and MMEs formed by mixing of coeval magma.Zircon Ti thermometers and oxygen fugacity of the host granodiorite and the MMEs show high oxygen fugacity,similar to that of W-Cu (Mo) mineralized granitoids in the eastern Jiangnan orogenic belt.A similar magma mixing process was probably one of the mechanisms that generated the W-Cu (Mo) fertile melts.     

广东新丰新坪花岗闪长岩(104 Ma)地球化学、锆石U-Pb年龄和Hf同位素研究

新坪铜钼矿化花岗闪长岩体位于佛冈复式岩体中北部,LA-ICP-MS锆石U-Pb年龄为（104.6±1.8）Ma（MSWD=1.2）,侵位时间为早白垩世晚期。岩石呈中酸性（SiO₂=63.43%~65.27%）、准铝质（ACNK=0.76~0.92）、中等碱含量（ALK=6.09%~7.63%）和富钾钙碱性特征,富集轻稀土,LREE/HREE=8.26~13.20,（La/Yb）_N=8.17~16.64,轻微的铕亏损（δEu=0.73~0.87）,富集Rb、U、Th和La,亏损Nb、Ta、Ti和P、Ba、Sr。锆石ε_Hf（t）值为-3.0~0.2,Hf二阶段模式年龄t_DM2=1.16~1.36Ga,镁指数（Mg^#）=42.64~46.95,表明源岩主要为中元古代地壳物质,并有亏损地幔组分的参与。Nb/U和Nb/La比值分别为0.20~0.38和1.7~2.2,显示源区受古太平洋板块俯冲流体交代作用的影响。研究表明,新坪岩体成岩过程主要受部分熔融作用控制,形成于燕山晚期古太平洋板片俯冲消减作用下的伸展拉张环境,与玄武质岩浆底垫诱发中元古代结晶基底的部分熔融有关。早白垩世晚期为中国东南部晚中生代重要的Cu-Au-Mo成矿期,位于南岭中东段的新坪花岗闪长岩体与邻区闽西南紫金山铜多金属岩（矿）体形成时代一致、岩石地球化学特征较为相似,且野外能观察到明显的铜钼矿化,因此具有较好的铜钼找矿前景。     

张广才岭地块早侏罗世晚期花岗闪长岩及其闪长质包体的岩石成因及构造意义

对松嫩-张广才岭地块早侏罗世晚期北长岗花岗闪长岩及其暗色包体开展了详细的岩石学,地球化学及年代学研究。结果表明,北长岗花岗闪长岩中含有大量闪长质暗色微粒包体,包体具有典型的岩浆结构,并发育针状磷灰石,同时斜长石斑晶从核部到边部显示复杂的成分和结构不平衡。寄主岩与包体均为准铝质,高钾钙碱性或高钾钙碱性-钙碱性过渡岩石,两者具有基本一致的稀土元素及微量元素组成,同时都具有正的εNd(t)和锆石εHf(t)值。主微量元素-SiO2协变图中,包体成分的两端分别向寄主花岗闪长岩及该区时代略早的基性岩延伸,三者具有良好的线性关系。以上特征表明北长岗花岗闪长岩中暗色包体为岩浆混合成因,且其原始岩浆可能为受到俯冲沉积物熔流体交代的软流圈地幔。寄主花岗闪长岩原岩主要为中-新元古代新生基性下地壳,并有少量亏损地幔物质的加入。寄主岩和暗色微粒包体具有基本一致的锆石U-Pb年龄(分别为173±2.0 Ma和173±1.5Ma),显示岩浆混合作用发生在早侏罗世晚期。区域构造演化显示,早侏罗世(～185 Ma),张广才岭地块处于古太平洋西-北西向俯冲到亚洲大陆之下形成的弧后伸展环境,北长岗花岗闪长岩及其暗色包体的形成暗示早侏罗世晚期(～173Ma)依然存在幔源岩浆活动,该区仍然处于弧后伸展环境,幔源物质的底侵诱发下地壳部分熔融形成研究区长英质岩浆,同时也说明该区早侏罗世晚期仍然存在地壳增生,且增生方式为垂向增生。     

大兴安岭中北段大金山地区早侏罗世埃达克质花岗闪长岩的成因及构造环境

对大兴安岭中北段大金山地区的花岗闪长岩进行了锆石LA-ICP-MS U-Pb年代学和岩石地球化学研究,以便了解其形成时代、成因及构造背景。结果表明,花岗闪长岩为早侏罗世(183. 2±1. 0 Ma)岩浆事件的产物,具有较高的SiO_2(>56%)、Al_2O_3(>15%)和较低的MgO(<3%),较高的Na_2O/K_2O值(1. 12～2. 04),为高钾钙碱性岩浆系列;具有较高的Sr含量(>400×10^-6),不明显的Eu异常,轻重稀土分异明显,强烈亏损Yb和Y;富集轻稀土元素,具有埃达克岩的地球化学特征。大金山岩体表现为富钾、低镁(Mg~#为38. 70)特征,属于C型埃达克质岩,推测其形成于加厚的玄武质下地壳的部分熔融。早侏罗世埃达克岩的发现表明大兴安岭中北段大金山地区有陆壳加厚的过程,大陆发生过垂向增生;大金山岩体可能形成于蒙古-鄂霍茨克洋俯冲作用下的活动大陆边缘环境。     

Paleo-Tethyan Oceanic Crust Subduction in the Eastern Section of the East Kunlun Orogenic Belt: Geochronology and Petrogenesis of the Qushi'ang Granodiorite

The Qushi'ang granodiorite(QSG) is located at the central east of the ophiolitic melange belt in the East Kunlun Orogenic Belt(EKOB) in the northern margin of the Qinghai-Tibetan Plateau. LA-MC-ICP-MS zircon U–Pb dating suggests that the granodiorite and mafic microgranular enclaves(MMEs) crystallized 246.61±0.62 and 245.45±0.9 Ma ago, respectively. Granodiorite, porphyritic diorite, and MMEs are metaluminous and medium-K calk-alkaline series, with island-arc magma features, such as LILE enrichment and HFSE depletion. The porphyritic diorite has high Cr(13.50 ppm to 59.01 ppm), Ni(228.53 ppm to 261.29 ppm), and Mg~#(46–54). Granodiorite and porphyritic diorite have similar mineral compositions and evolved major and trace elements contents, particularly Cr and Ni, both of which are significantly higher than that in granites of the same period. The crystallization age of MMEs is close to that of granodiorite, and their major and trace elements contents are in-between porphyritic diorite and granodiorite. The results suggest that the original mafic magma, which was the product of mantle melting by subduction process, intruded into the lower crust(Kuhai Rock Group), resulting in the formation of granodiorite. Countinous intrusion of mafic magma into the unconsolidated granodiorite formed MMEs and porphyritic diorite. The granodiorite reformed by late-stage strike-slip faulting tectonic event indicates that the strike-slip fault of Middle Kunlun and the collision of the Bayanhar block with East Kunlun were later than 246 Ma. Therefore, the formation of the QSG not only indicates the critical period of evolution of East Kunlun but also represents the tectonic transition from oceanic crust subduction to slab breaking.     

Petrogenesis and W-Mo fertility indicators of the Gaojiabang “satellite” granodiorite porphyry in southern Anhui Province,South China

The Gaojiabang W-Mo mineralized granodiorite porphyry occurs as a “satellite” intrusion on the northern margin of the Qingyang-Jiuhua pluton in southern Anhui Province, South China. The geology, mineral and whole-rock geochemistry of the granodiorite porphyry is I-type and formed from a reduced melt. The Gaojiabang mineralized granodiorite porphyry was derived from the transition zone between the lower crust and upper mantle, and underwent high-degree fractional crystallization during emplacement. There is no obvious genetic relationship between the granodiorite porphyry and the granodiorite pluton of the Qingyang-Jiuhua plutonic complex. The key characteristics of the W-Mo mineralized Gaojiabang granodiorite porphyry include: 1) small volume (<0.5 km³), 2) an abundance of hornblende (>5 vol%), 3) low ∑REE concentrations (145.50–159.46 ppm) accompanying weak negative Eu anomalies (0.85–0.94), 4) high differentiation-indices degree and water contents, and 5) low oxygen fugacity. These criteria can be used to identify potential W-Mo mineralized intrusions in southern Anhui Province, South China.     

柴北缘牛鼻子梁地区花岗闪长岩锆石U-Pb年代学及地球化学研究

柴北缘牛鼻子梁地区的花岗闪长岩锆石U-Pb年代学及岩石地球化学研究结果表明,该花岗闪长岩的锆石LA-ICP-MS U-Pb谐和年龄为393.2~395.6Ma,对应的加权平均年龄为(394.4±2.4)Ma,表明该岩石形成于中泥盆世。岩石主量元素具有较高的SiO_2(65.70%~69.22%)和Al_2O_3(14.96%~16.03%)、较低的MgO(1.37%~1.89%)含量,A/CNK值为0.95~0.98,属于钙碱性、准铝质系列,具有I型花岗岩的特征,表明其岩浆起源于下地壳部分熔融。岩石稀土总量较高,无明显的Eu异常(δEu=0.95~1.30),富集轻稀土元素和K、Rb、Ba、U等大离子亲石元素,相对亏损重稀土元素和Nb、Ta、Ti、Th、Zr等高场强元素。综上可知,岩石具有碰撞后花岗岩的地球化学特征属性,结合区内晚泥盆世牦牛山组陆相火山岩和区域构造演化特征,认为牛鼻子梁地区的花岗闪长岩形成于柴达木地块与南祁连地块碰撞后的伸展环境。     

欧龙布鲁克地块西北缘青海冷湖北片麻状花岗闪长岩锆石U-Pb年龄、地球化学特征及地质意义

欧龙布鲁克地块西北缘青海冷湖北片麻状花岗闪长岩SiO_2、MgO含量较低,Al_2O_3、Na_2O、CaO含量较高,铝饱和指数(ASI)为0.79~0.92,属高钾钙碱性系列。球粒陨石标准化稀土元素配分曲线表现为轻稀土元素富集型,轻、重稀土元素分馏明显,Eu呈弱的负异常(δEu=0.82~0.90)。微量元素地球化学特征显示,岩石富集大离子亲石元素Rb、Th和U,弱亏损高场强元素Nb、Ta、Zr等,显示出与板块俯冲作用有关的造山带岩浆岩的特征。青海冷湖北片麻状花岗闪长岩为角闪岩相压力条件下变玄武质岩石部分熔融的产物。LA-ICP-MS锆石206Pb/238Pb年龄为470.2±1.2Ma(n=19,MSWD=0.68),代表片麻状花岗闪长岩的成岩年龄。结合区域地质演化特征认为,该岩体形成于火山弧构造环境,是早奥陶世北阿尔金洋壳向欧龙布鲁克地块俯冲作用的地质记录。     

粤北大宝山矿区花岗闪长斑岩LA-ICP-MS锆石U-Pb年龄及其地质意义

对粤北大宝山钼钨多金属矿区的花岗闪长斑岩进行了锆石阴极发光显微结构观察和LA-ICP-MS锆石微区U-Pb定年.在锆石阴极发光图像中, 两个花岗闪长斑岩样品中的锆石颗粒具有明显的振荡环带, 为典型的岩浆锆石.LA-ICP-MS U-Pb定年结果显示: 大宝山花岗闪长斑岩样品中具岩浆环带锆石区域12个分析点的206Pb/238U加权平均年龄为175.8±1.5 Ma(MSWD=0.037), 船肚花岗闪长斑岩样品中具岩浆环带锆石区域16个分析点的206Pb/238U加权平均年龄为175.0±1.7 Ma(MSWD=0.41).它们的形成时代约175 Ma, 为燕山早期第Ⅰ阶段(180~170 Ma)产物.本次对大宝山矿区花岗闪长斑岩形成时代的准确厘定, 为大宝山和船肚花岗闪长斑岩原本连为一体被后期构造错断的认识提供了新证据, 岩体错断部位是矿区Mo-W矿进一步勘探的重要方向.结合相关研究表明, 其成岩成矿动力学背景与邻区岩体相似, 为南岭地区同一期岩石圈伸展-减薄事件的产物.同时启示我们, 在南岭地区还存在燕山早期第一阶段(180~170 Ma)利于形成Mo-W矿的花岗岩类小岩体未被揭露.      

鲁西蒙山龟蒙顶、云蒙峰岩体的锆石SHRIMP U-Pb测年及形成时代

蒙山主峰龟蒙顶为片麻状花岗闪长岩,原划为新太古代早期侵入岩。根据新测的2件样品锆石SHRIMP U-Pb年龄分别为(2 539±17)Ma,(2 544±15)Ma,其形成时代确定为新太古代晚期。蒙山北部云蒙峰为中粒二长花岗岩,锆石SHRIMP U-Pb年龄为(2 534±8)Ma,其侵入龟蒙顶岩体片麻状花岗闪长岩。龟蒙顶岩体属峄山岩套TTG质花岗岩,是地幔岩浆侵入混入地壳物质形成的。云蒙峰岩体属傲徕山岩套二长花岗岩,为上地壳物质重熔(深熔)作用形成。