甘肃金川早古生代正长花岗岩锆石SHRIMP U-Pb年代学、岩石学、地球化学特征及其构造意义

报道了甘肃金川正长花岗岩体的岩石学、锆石U-Pb年代学和全岩地球化学资料,以确定岩体的形成时代、岩石成因及其构造属性.正长花岗岩样品中岩浆锆石SHRIMP U-Pb加权平均年龄为(425.7±2.5)Ma(MSWD=1.07),属晚志留世,与金川地区出露的辉绿岩脉年龄相近.正长花岗岩低硅(SiO266.71％～70.0...     

北山柳园地区双峰山早泥盆世A型花岗岩的确定及其构造演化意义

北山柳园地区分布有大量的早中古生代花岗岩类岩石.柳园双峰山岩体具有高硅、高碱(AR=3.99～5.05,NK/A>0.85)、高FeOT/MgO比值和10 000×Ga/Al值、低Al2O3、贫CaO和MgO的特征,显示出准铝质、碱质花岗岩的特点;∑REE较高,LREE略富集,轻重稀土元素分馏不十分明显,Eu负异常明显;相对富集Rb、K、Pb等大离子亲石元素(LILE),强烈亏损Ba、Sr、P、Eu、Ti,弱亏损Ta、Nb等元素;同时具有较高的Rb/Nb和Y/Nb比值,显示了A2型铝质花岗岩的特征.采用LA-ICP-MS锆石U-Pb定年方法,获得双峰山岩体的206Pb/238U年龄为415±3 Ma(MSWD=1.5),代表该岩体的形成年龄,即双峰山岩体形成于早泥盆世.地球化学及Nd同位素特征综合分析显示,该岩体可能由幔源岩浆底侵导致上覆地壳物质(可能由洋壳和岛弧建造组成)部分熔融形成的花岗闪长质岩浆经进一步结晶分异作用形成,为该区较早的钙碱性花岗岩演化到后期的产物.岩体特征、年代学、地球化学和地质背景综合分析结果表明,该岩体形成于后造山或造山作用演化晚期阶段.双峰山早泥盆世A型花岗岩为目前北山地区发现的最老的A型花岗岩,这对探讨古生代花岗岩成因类型及岩浆演化具有重要的意义.     

Early Mesozoic tectonic transition of the eastern South China Block: constraints from Late Triassic Dashuang complex in eastern Zhejiang Province

ABSTRACT

The Mesozoic tectonic transition from the Palaeo-Tethys tectonic regime to the Palaeo-Pacific tectonic regime in the eastern South China Block has long been debated. Geochemical and zircon U–Pb–Hf isotopic studies were conducted on the Dashuang complex in the eastern Zhejiang Province. The Dashuang complex consists mainly of quartz syenite in the northwestern part and quartz monzonite in the southeastern part. New laser ablation inductively coupled plasma mass spectrometry zircon U–Pb data show that the quartz syenite, the quartz monzonite, and its chilled margin (fine-grained granite) crystallized at 235 ± 4 Ma, 232 ± 3 Ma, and 230 ± 1 Ma, respectively. The Dashuang complex intrudes into the Chencai Group gneiss that postdated ~646 Ma and underwent anatexis at 443 ± 14 Ma. The quartz monzonite shows A-type granite affinity, characterized by high K₂O + Na₂O and Zr + Nb + Ce + Y, high FeO_T/(MgO + FeO_T) and Ga/Al ratios, an enrichment in light rare earth elements, and depletions in Ba, Sr, and Eu. The quartz monzonite has zircon ε_Hf(t) values of ?14.2 to –11.9 and two-stage model ages of 1788–1922 Ma. Zircon ε_Hf(t) values of the chilled margin (fine-grained granite) and wall rock (gneiss) are scattered (?18.2 to –6.3 and ?19.5 to 10.2). The corresponding two-stage model ages are 1482–2133 Ma and 1184–2471 Ma, respectively. The Dashuang complex was derived mainly from partial melting of Neoproterozoic clastic rocks in the Cathaysia Block. Geochemical data indicate that the quartz monzonite formed in a post-collision extensional environment. These results, considered with previous data, indicate that the transition from the Palaeo-Tethys to the Palaeo-Pacific tectonic regimes of the eastern South China Block occurred during the Late Triassic (225–215 Ma).     

中国东南部晚中生代构造伸展作用——来自海南岛基性岩墙群的证据

论文在系统地分析了海南岛文市、叉河、三亚地区的三处基性岩墙群的LA-ICP-MS锆石U-Pb年代学和元素地球化学特征的基础上,探讨了岩墙群的来源和其反映的地球动力学背景。文市岩墙群形成于约101Ma,叉河和三亚岩墙群形成于约93Ma;主量元素化学特征显示它们属碱性系列,微量元素化学特征为富集轻稀土元素和大离子亲石元素(Sr、K、Rb、Ba、Th),亏损高场强元素(Nb、Ta、Ti);Sr-Nd同位素组分显示其源区具有EMⅡ特征。这两期岩墙群的存在,显示了海南岛地区在早白垩世晚期和晚白垩世早期存在两次构造伸展事件。海南岛90Ma左右的岩墙群与广东、福建同时代的岩墙群构成中国东南沿海90Ma广泛发育的、呈近北北东向展布的岩墙群带,它们具有相同成因机制,指示中国东南部在90Ma左右时经历了强烈区域性拉张作用;这些岩墙群虽来源于不同的地幔源区,但均与俯冲流体交代作用有关。     

华北克拉通减薄的岩浆岩响应:来自河北武安洪山含霓石斑状正长岩的证据

洪山碱性杂岩体出露于河北省邯郸市永年县,地处太行山南段,出露面积约53 km²,岩性以正长岩类为主体,侵位于晚古生代—中生代地层中。洪山碱性杂岩体中正长岩属于钾质的碱性系列岩石,形成于拉张环境,其形成年龄为(120.1±4.6) Ma,是华北克拉通岩浆作用峰期的产物。岩体稀土元素配分图中无Eu异常,表现为轻稀土元素富集的配分型式。富集Rb、K、Sr等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素,具有大陆地壳特征。野外观察到岩石具有从深成到浅成-超浅成的特征,电子探针发现长石出现反环带,正长岩中不含继承锆石等特征,结合若要使地壳内岩石部分熔融产生无铕异常的正长质岩浆,那么压力应大于20 kbar,说明洪山碱性杂岩体形成于加厚地壳局部熔融,后经历了少量幔源岩浆混合及地幔流体的注入等过程,导致正长岩出现一定的地幔Hf、Nd同位素印记,而未改变其主微量元素特征,随后在拉张环境下快速上升至地表浅处。     

苏鲁造山带胶东区段花岗片麻岩类的常量与微量元素地球化学：扬子克拉通北缘新元古代活动大陆边缘的证据

胶东东部地区的基底片麻岩以牟平-海阳断裂为界,两侧在岩性组合和地球化学性质上完全不同,本文认为该断裂应代表苏鲁造山带的西北边界（北段）。断裂西侧岩性包括花岗闪长质、奥长花岗质和花岗质,地球化学上表现为低硅（SiO2：64．16～71．76％）、高铝（Al2O3=15．60-18．51％）、显著的Ba和Sr正异常、强烈的稀土元素分馏程度（LaN／YbN=15．77～68．19）和贫重稀土元素（YbN=2．9-4．4）、Eu异常不明显（δEu=0．86～1．02）,具典型的太古代高铝TTG岩石组合的特征。而东侧新元古代的基底片麻岩从地球化学上可分为高钾的Ⅰ型花岗质片麻岩和富钠的A型花岗质片麻岩。前者总体表现为富钾（K2O／Na2O比接近或大于1）和亚铝质,强烈亏损Sr、Nb和Ta,轻稀土元素之间的分馏程度强（LaN／SmN=4．21～5．37）,而重稀土元素之间几乎无分馏（GdN／YbN：0．78～1．54）,负Eu异常较强（δEu=0．47～0．61）,岩石地球化学特征显示它们的原岩类似于活动大陆边缘的Ⅰ型花岗岩类。而后者则以偏碱和富钠（Na2O／K2O=1．06～1.77）,高Y（20.4～52．9μg／g）、Zr（218～39μg/g）、Ga（18．8～22．9μg／g）及稀土元素丰度为特征,稀土元素分馏程度强（LaN／YbN=11．30～19．09）,弱到中等程度的负Eu异常（δEu=0．94～0．65）,强烈亏损Sr为显著特征,Nb和Ta相对于La也强烈亏损,而Ba则表现出明显的正异常。推测Ⅰ型花岗岩是在与俯冲有关的构造环境下,压力0．8～1．0Gpa条件下,由受俯冲板片脱水交代的镁铁质下地壳（26～33km）部分熔融形成的。而A型花岗岩则是在Ⅰ型花岗质岩浆形成后,由脱水的紫苏辉石质残留下地壳在温度大于900℃的条件下再一次部分熔融形成的。相对于Ⅰ型花岗岩,A型花岗岩中可能有更多的大洋岩石圈的组份被卷入。     

内蒙古双尖子山银多金属矿床花岗斑岩年代学、地球化学特征及构造意义

本文对内蒙古双尖子山银多金属矿床花岗斑岩进行了锆石SHRIMP U-Pb定年,并对花岗斑岩的主量元素、微量元素组成进行了分析研究。结果显示,双尖子山花岗斑岩加权平均年龄为(133.4±1.2)Ma,为大兴安岭南段早白垩世岩浆活动集中期产物;花岗斑岩SiO_2含量为68.81%~70.05%,低MgO、TiO_2,K_2O/Na_2O为0.97~1.27,较为富钾;A/CNK在0.94~1.12之间,属准铝-弱过铝系列;富集Rb、Th、U、K等大离子亲石元素(LILE),亏损Sr、P、Ti等高场强元素(HFSE);稀土总量ΣREE中等,δEu为0.55~0.62,Eu中等负异常。本文认为双尖子山银多金属矿床为多期次叠加的岩浆热液型矿床,160Ma和133Ma可能分别是岩浆活动的两个高峰期。花岗斑岩形成于陆内晚造山期-造山期后的伸展环境,蒙古-鄂霍次克洋俯冲板片在深部断离,引发软流圈物质上涌,这是导致大兴安岭南段早白垩世岩石圈伸展环境的主要机制。     

Field evidence,mineral chemical and geochemical constraints on mafic-felsic magma interactions in a vertically zoned magma chamber from the Chotanagpur Granite Gneiss Complex of Eastern India

The Nimchak granite pluton (NGP) of Chotanagpur Granite Gneiss Complex (CGGC), Eastern India, provides ample evidence of magma interaction in a plutonic regime for the first time in this part of the Indian shield. A number of outcrop level magmatic structures reported from many mafic-felsic mixing and mingling zones worldwide, such as synplutonic dykes, mafic magmatic enclaves and hybrid rocks extensively occur in our study domain. From field observations it appears that the Nimchak pluton was a vertically zoned magma chamber that was intruded by a number of mafic dykes during the whole crystallization history of the magma chamber leading to magma mixing and mingling scenario. The lower part of the pluton is occupied by coarse-grained granodiorite (64.84–66.61?wt.% SiO₂), while the upper part is occupied by fine-grained granite (69.80–70.57?wt.% SiO₂). Field relationships along with textural and geochemical signatures of the pluton suggest that it is a well-exposed felsic magma chamber that was zoned due to fractional crystallization. The intruding mafic magma interacted differently with the upper and lower granitoids. The lower granodiorite is characterized by mafic feeder dykes and larger mafic magmatic enclaves, whereas the enclaves occurring in the upper granite are comparatively smaller and the feeder dykes could not be traced here, except two late-stage mafic dykes. The mafic enclaves occurring in the upper granite show higher degrees of hybridization with respect to those occurring in the lower granite. Furthermore, enclaves are widely distributed in the upper granite, whereas enclaves in the lower granite occur adjacent to the main feeder dykes.Geochemical signatures confirm that the intermediate rocks occurring in the Nimchak pluton are mixing products formed due to the mixing of mafic and felsic magmas. A number of important physical properties of magmas like temperature, viscosity, glass transition temperature and fragility have been used in magma mixing models to evaluate the process of magma mixing. A geodynamic model of pluton construction and evolution is presented that shows episodic replenishments of mafic magma into the crystallizing felsic magma chamber from below. Data are consistent with a model whereby mafic magma ponded at the crust-mantle boundary and melted the overlying crust to form felsic (granitic) magma. The mafic magma episodically rose, injected and interacted with an overlying felsic magma chamber that was undergoing fractional crystallization forming hybrid intermediate rocks. The intrusion of mafic magma continued after complete solidification of the magma chamber as indicated by the presence of two late-stage mafic dykes.     

胶东半岛燕山期变质作用——来自苏鲁构造杂岩带乳山地区(石榴)斜长角闪岩锆石与榍石U-Pb测年的新证据

“燕山运动”泛指中国东部侏罗纪—白垩纪大规模构造运动,与之相关的构造作用及岩浆作用前人已有深入的研究,然而对于燕山运动相关变质作用却鲜有报道。通过对胶东半岛苏鲁构造杂岩带乳山地区（石榴）斜长角闪岩进行锆石及榍石U-Pb测年,发现念头石榴斜长角闪岩原岩时代为1708 Ma,变质时代为154 Ma;北庄斜长角闪岩早期变质时代为1840 Ma,晚期变质时代为145 Ma。综合上述年代学数据与近年有关胶东半岛燕山期构造-岩浆事件新进展,表明乳山地区（石榴）斜长角闪岩记录了燕山期变质作用,推测乳山地区燕山期变质作用发生于中国东部构造转换期（挤压到伸展）,其成因可能与中国东部燕山造山过程中区域构造增厚作用有关,但目前不能排除该期变质作用可能与同期岩浆作用热驱动有关。     

赣南部分岩体的锆石铀-铅同位素年代学研究及其对成岩成矿机制的再认识

赣南岩浆岩分布广泛,有的岩体成岩时代明确,有的则存在争议。利用激光多接收器电感耦合等离子体质谱(LA-MC-ICPMS)技术测定了田新、路迳、水头、白鹅、高山角、韩坊、杨村、湖南洞、珠兰埠、淋洋、大富足、油山、吉埠黄沙、阳埠和万田等岩体的U-Pb年龄,表明这些岩体不全是燕山期形成的,即使是燕山期的田新、路迳、水头、白鹅、韩坊、杨村等岩体中也存在有更早期的锆石,大部分岩体经历了6～10 Ma的结晶成岩历史。通过与以往K-Ar、Rb-Sr等时线方法的初步对比,指出赣南地区花岗岩虽然在同位素年龄数据上以侏罗纪为多,但印支期的岩体面积不小,前寒武纪和加里东期的信息也不断增多,说明赣南花岗岩型地壳的演化经历了漫长的历史过程,相当一部分岩体不是单一成岩期形成的,而是多期次多阶段形成的。加里东期的岩体以往不被注意,认为与钨矿关系不密切,但可以成为离子吸附型稀土矿的成矿母岩。     

河南东沟钼矿花岗斑岩成因: 岩石地球化学、锆石U-Pb年代学及Sr-Nd-Hf同位素制约

本研究对东沟超大型钼矿床的成矿母岩-东沟花岗斑岩开展了系统的年代学、岩石地球化学及Sr-Nd-Hf同位素分析工作.LA-ICP-MS锆石U-Pb定年结果表明,东沟花岗斑岩成岩年龄为114～117Ma,与已有的成矿年龄(116±2Ma,Re-Os法)一致,证实了东沟钼矿为一斑岩型矿床.详细的岩石地球化学分析显示东沟花岗斑岩岩体与区域上太山庙大型花岗岩基为同源演化关系,它们均为弱过铝质,具有富Si、富K、富Rb、Th、U等大离子亲石元素、富Nb、Ta、Zr、Hf等高场强元素,贫Fe、Mg、Ca,贫Sr、Ba,Ga/Al比值较高等地球化学特征,属铝质A型花岗岩,形成于伸展构造体制,东沟岩体是母岩浆经历了强烈结晶分异高度演化的产物;东沟岩体Nd同位素组成为0.51166～0.51182,ε_(Nd)(t))值在-17.3～-14.3之间,锆石的ε_(Hf)(t)值变化较大,由-3.4至-18.7,另有一颗年龄为1715Ma的捕获锆石的ε_(Hf)(t)值为-2.4,Nd、Hf模式年龄分别为1.5～1.8Ga与1.3～1.7Ga.我们认为东沟岩体的岩浆源区以古老地壳物质为主,但也有少量幔源组分参入,并且幔源物质的加入及很高的岩浆演化程度可能对东沟钼矿的成岩成矿过程具有重要作用.     

Mid-Cretaceous granitic magmatism during the transition from subduction to extension in southern New Zealand: a chemical and tectonic synthesis

Regional geochronological studies indicate that mid-Cretaceous plutonism (the Hohonu Suite at 110 Ma) in the Hohonu Batholith, Western Province of New Zealand, occurred during a period of rapid tectonic change in the SW Pacific portion of Gondwana. The 30–40 m.y. preceding Hohonu Suite magmatism were dominated by the subduction-related plutonism of the Median Tectonic Zone volcanic arc. Between 125–118 Ma there was a major collisional event, inferred to be the result of collision between the Median Tectonic Zone and the Western Province. This collision resulted in melting of the Median Tectonic Zone arc underplate and generation of a distinctive suite of alkali-calcic granitoids, termed the Separation Point Suite. At 110 Ma there was another pulse of magmatism, restricted to the Buller terrane of the Western Province, and including the Hohonu Suite granitoids. This was followed almost immediately by extension, culminating in the opening of the Tasman Sea some 30 m.y. later. The Hohonu Suite granitoids overlap temporally with the last vestiges of collisional Separation Point magmas and the onset of crustal extension in the Western Province, and thus represent magmatism in a post-collisional setting. Hohonu Suite magmas are typically calc-alkaline, but retain a chemical signature which suggests that the earlier Separation Point Suite magmas and/or sources were involved in Hohonu Suite petrogenesis. A model is proposed in which rapid isothermal uplift, resulting from the post-collisional collapse of continental crust previously thickened during the Median Tectonic Zone collision, caused melting of lower continental crust to generate the Hohonu Suite granitoids. In this example, granitoid composition is a consequence of the composition of the source rocks and the conditions present during melting, and no geochemical signature indicative of the tectonic setting during magmatism is present.