Timing of Magma Mixing in the Gangdisě Magmatic Belt during the India-Asia Collision： Zircon SHRIMP U-Pb Dating

Abstract  Abundant mafic microgranular enclaves (MMEs) extensively distribute in granitoids in the Gangdisê giant magmatic belt, within which the Qüxü batholith is the most typical MME‐bearing pluton. Systematic sampling for granodioritic host rock, mafic microgranular enclaves and gabbro nearby at two locations in the Qüxü batholith, and subsequent zircon SHRIMP II U‐Pb dating have been conducted. Two sets of isotopic ages for granodioritic host rock, mafic microgranular enclaves and gabbro are 50.4±1.3 Ma, 51.2±1.1 Ma, 47.0±1 Ma and 49.3±1.7 Ma, 48.9±1.1 Ma, 49.9±1.7 Ma, respectively. It thus rules out the possibilities of mafic microgranular enclaves being refractory residues after partial melting of magma source region, or being xenoliths of country rocks or later intrusions. Therefore, it is believed that the three types of rocks mentioned above likely formed in the same magmatic event, i.e., they formed by magma mixing in the Eocene (c. 50 Ma). Compositionally, granitoid host rocks incline towards acidic end member involved in magma mixing, gabbros are akin to basic end member and mafic microgranular enclaves are the incompletely mixed basic magma clots trapped in acidic magma. The isotopic dating also suggested that huge‐scale magma mixing in the Gangdisê belt took place 15–20 million years after the initiation of the India‐Asia continental collision, genetically related to the underplating of subduction‐collision‐induced basic magma at the base of the continental crust. Underplating and magma mixing were likely the main process of mass‐energy exchange between the mantle and the crust during the continental collision, and greatly contributed to the accretion of the continental crust, the evolution of the lithosphere and related mineralization beneath the portion of the Tibetan Plateau to the north of the collision zone.     

哈密头苏泉哈尔欣巴A型花岗岩厘定

初步研究表明新疆哈密头苏泉地区哈尔欣巴花岗岩为A型花岗岩.该岩体富硅(SiO2=71.87%~76.80%)和碱(K2O+N2O=7.39%~8.94%),贫铁、锰、镁(FeOt 0.67%~2.04%;MnO 0.03%~0.06%;MgO 0.10%~0.61%),具较高FeOt/MgO比值,A/NKC=0.78~1.02,属准铝质花岗岩石.在微量元素和稀土元素组成上,岩石富Zr,Rb,Ce等不相容元素,亏损Ni,Co,Cr等元素.10000×Ga/Al为3.12~4.1,大于A型花岗岩下限值(2.6).在Zr,Ce,Nb,Y与10000×Ga/Al,及(Nb+Zr+Ce+Y)/(FeOt/MgO)、SiO2/( FeOt/MgO)图解中大多数点都落在A型花岗岩区域.在A1-A2构造环境判别图上显示后造山花岗岩特征.对头苏泉地区哈尔欣巴A型花岗岩的厘定,为研究该区地壳物质组成及构造演化具重要意义.     

江南造山带新元古代中期(830～750Ma)岩浆活动及对构造演化的制约

连接扬子地块和华夏地块的江南造山带是华南前寒武纪最重要的构造单元,其形成和演化长期以来备受关注。在江南造山带的范围内广泛发育了新元古代岩浆岩,它们是探讨江南造山带构造演化的重要对象,但其成因和形成的构造背景却备受争论。本文系统收集和分析了江南造山带830~820 Ma花岗岩、800~780 Ma酸性岩和800~750 Ma基性岩的地球化学数据。研究表明,不同时间段的岩石成因类型存在系统差异,830~820 Ma的花岗岩主要为S型花岗岩,800~780 Ma的酸性岩主要为A型酸性岩,而800~750 Ma的基性岩以拉斑系列和碱性系列为主,并在构造判别图中显示了板内玄武岩(WPB)和洋中脊玄武岩(MORB)的特征。综合同位素年代学、岩石地球化学和沉积学等学科领域的研究成果我们认为:扬子北缘和西缘应先于东南缘在1000~900 Ma期间发生碰撞,而此时的东南缘仍为活动大陆边缘;直到~830 Ma,扬子地块与华夏地块沿江南造山带发生拼贴,但只是陆-弧-(微)陆之间的"软碰撞",而无山脉隆升和高级变质作用,各个块体之间处于"联而不合"的状态,大洋岩石圈拆沉之后的软流圈上涌和由拆沉所引起的拉张作用导致了上覆岩石圈和陆壳发生部分熔融,产生了江南造山带830~820 Ma的S型花岗岩;随着全球Rodinia超大陆的裂解,~820 Ma,华南裂谷盆地开启,并在随后的裂解过程中发育了大量与伸展有关的800~780 Ma A型酸性岩和基性岩脉/墙,而其明显高于同时代MORB源区的地幔潜能温度显示,导致Rodina超大陆裂解的地幔柱可能对该时期岩浆岩的地幔源区有一定影响;随着拉张作用的不断加强,出现了760~750 Ma碱性系列和具MORB特征的基性岩,此时的软流圈地幔既提供热量又有物质供应。     

Interaction of crustal and mantle materials,sources of trace elements during the formation and evolution of Early Paleozoic Li-rich granite-pegmatite systems in southeastern Tuva

We present new data on the age, composition, and environments of formation of granites of the Kystarys complex and the associated Li-rich rare-element pegmatites of the South Sangilen pegmatite belt including the large Tastyg lithium deposit. It has been established that they formed during the Early Paleozoic collisional orogeny in the Tuva-Mongolian massif at the Cambrian-Ordovician boundary. The granites of the Kystarys complex are moderately alkaline high-K rocks and are enriched in Zr, Nb, Y, and REE; therefore, they are classified as postcollisional, transitional to within-plate (A-type). The spodumene pegmatites of the South Sangilen pegmatite belt are similar to the above granites in age and isotopic and geochemical parameters, which suggests a paragenetic relationship between these rocks. Pegmatites form several pegmatite fields within the belt, which differ in trace-element signatures. In addition to predominant Li, Cs, and Ta, specific to all spodumene pegmatites (LCT family), pegmatites of two fields have high contents of Nb, Y, REE, and Zr, which are indicator elements of NYF family pegmatites. It has been established that the formation of spodumene pegmatites with combined LCT-NYF geochemical signatures was preceded by the intrusion of dikes of monzogabbro with the geochemical characteristics of OIB and of alkali aegirine granites and by the formation of associated metasomatites enriched in Zr, Nb, Y, and REE. Based on the geological, mineralogical, and geochemical data, we substantiate the hypothesis of the formation of Li-bearing granite-pegmatite melts from a mixed source resulted from the influence of fluids of an alkaline igneous complex of mantle genesis on the crustal protolith.     

朝鲜半岛平南盆地中元古代岩浆事件

朝鲜平南盆地翁津地区发育中元古代黄海群和同时期(称之为瓮津期)花岗岩,花岗岩体侵入于黄海群。本文采用锆石原位微区U-Pb定年技术,对黄海群中的酸性火山岩及花岗岩进行了年龄测试。获得的数据表明,黄海群中下部层位及上部层位的酸性火山岩分别在1235±5Ma和1203±7Ma喷发,由此说明黄海群的沉积时代应为中元古代,而不是传统上认为的古元古代;两个翁津期花岗岩体(翁津和黄衣山岩体)的侵位年龄分别为1251±22Ma和1248±13Ma,为中元古代花岗质岩浆活动的产物。上述1251~1203Ma年龄的获得,表明朝鲜半岛发育中元古代岩浆作用,从而明确朝鲜黄海裂谷与华北东缘裂谷在时间上具有同期性,同时也表明中国华北与朝鲜在中元古代具有类似的发展历史。     

两广交界地区壶垌片麻状复式岩体的年代学和地球化学：对云开地块北缘早古生代构造-岩浆作用的启示

本文对两广交界地区发育的壶垌片麻状复式岩体进行了LA-ICP-MS锆石U-Pb定年和岩石学、地球化学、Sr-Nd-Pb同位素的分析研究。该复式岩体主要由片麻状英云闪长岩、片麻状花岗闪长岩和片麻状二长花岗岩组成,获得片麻状花岗闪长岩LA-ICP-MS锆石U-Pb谐和年龄为443.1±2.0Ma。其中片麻状英云闪长岩和片麻状花岗闪长岩总体具有较低硅(SiO2=62.92%~67.54%)、较低碱(K2O+Na2O=3.98%~5.17%)、准铝质(A/CNK=0.83~0.93)的化学组成特征,属于中钾含角闪石钙碱性花岗岩类(ACG);而片麻状二长花岗岩具有高硅(SiO2=71.55%~72.78%)、高碱(K2O+Na2O=6.65%~7.57%)、准铝-弱过铝质(A/CNK=0.92~1.05)的化学组成特征,属于高钾钙碱性花岗岩类(KCG)。岩石表现出富集大离子亲石元素(如U、Ba、Rb和Th)和轻稀土元素,而Nb、Ta和Ti等高场强元素和重稀土元素明显亏损,并具有较高的锶同位素初始比值((87Sr/86Sr)i=0.71268~0.71482)和较低的εNd(t)值(-9.4~-2.6),反映其具有俯冲消减作用形成的岛弧岩浆岩地球化学特征。结合区域地质特征分析认为,壶垌片麻状复式岩体很可能是在扬子板块和华夏板块之间的洋壳岩石圈向南俯冲的地球动力学背景下,引发软流圈地幔上涌,其所带来的热能诱发了岩石圈地幔和上覆云开地块的古老地壳物质重熔,形成以壳源为主的壳幔混源母岩浆,再经历不同程度分离结晶作用,从而形成了本区大陆边缘弧型岩浆岩。因此,壶垌片麻状复式岩体是云开地块北缘早古生代洋陆俯冲-碰撞的地质记录。     

内蒙古苏尼特左旗南两类花岗岩同位素年代学及其构造意义

内蒙古北部索伦缝合带（索伦-苏尼特左旗-锡林浩特）被多数中外学者认为是西伯利亚板块南缘和华北板块北缘的最终缝合带,本文选择该缝合带上苏尼特左旗南两类花岗岩-与俯冲有关和与碰撞有关的岩浆岩（分别叫弧岩浆岩和碰撞岗岩）进行同位素年代学研究,结果表明：（1）弧岩浆活动有两期,分别约为490Ma和310Ma(锆石U-Pb,SHRIMP);碰撞花岗岩的侵位年代在230-250Ma(Rb-Sr全岩和锆石U-Pb);（2）根据本文新的年代学数据,索伦缝合带的最终缝合时间可能是在230-310Ma,这显然不同于国内多数学者坚持的“晚泥盆世碰撞”模式;而Sengor等推测的“晚二又开展碰撞”模式与本文数据一致。     

胶东地区燕山期岩浆活动及其构造环境——来自单颗锆石SHRIMP年代学的记录

山东胶东地区中生代构造-岩浆事件和金矿成矿作用密切相关,其主要构造-岩浆事件包括:1220~200 Ma,扬子板块相对华北板块的南北向碰撞形成苏鲁高压-超高压变质带及同造山花岗岩与造山后高碱正长岩,属于典型的幔源型花岗岩系列;2 165~150 Ma,晚侏罗世时期形成与金矿有关的玲珑(昆嵛山)造山早期片麻状二长花岗岩组合,为过铝质花岗岩类,属胶东基底岩系的部分重熔产物。3 135~110 Ma,由壳幔混合岩浆结晶分异形成郭家岭花岗岩和伟德山花岗闪长岩。燕山期是胶东地区岩浆活动的鼎盛时期,表现为四次强烈的构造-岩浆事件,代表了燕山造山事件四个构造幕,并以挤压与伸展相互转化为特征。研究表明胶东地区中生代花岗岩具有继承性和再生性特点,采自郭家岭花岗闪长岩中的SHRIMP锆石U-Pb同位素年代学测定结果表明:锆石由核部到边部年龄值分别为2573~2194Ma、164.8~147.8和126.6~126.2Ma,显示了胶东结晶基底、玲珑花岗岩和郭家岭花岗岩形成的年代学信息,揭示了它们之间复杂的继承性和再生性,即新太古代胶东岩群、TTG岩系和古元古代荆山群、粉子山群等胶东基底岩系交代重熔形成S型玲珑花岗岩,由壳幔混合岩浆形成的郭家岭花岗岩在形成与侵位过程中重熔交代了部分玲珑花岗岩物质并侵位于玲珑花岗岩中。分析认为2573.4~2194.4 Ma是胶东结晶基底的年龄信息,164.8~147.8Ma代表了玲珑花岗岩形成的年龄信息,126.2~126.6Ma则代表了郭家岭花岗岩形成的年龄信息。这种年龄信息组合也反映了胶东地区从新太古代陆块形成、古元古代克拉通化到三叠纪华北陆块与扬子陆块碰撞至玲珑S型花岗岩形成、郭家岭壳幔混合型花岗岩形成演化的年代学记录,浓缩了胶东地区中生代岩浆起源和演化的过程,进而为诠释胶东地区深部岩浆作用过程提供了新的资料,并对后续的相关科学研究尤其是金矿的多期成矿作用提供了佐证。     

新疆西准噶尔沙勒克腾地区花岗岩锆石U-Pb年龄、地球化学特征及其对后碰撞构造环境的约束

以西准噶尔白杨河上游沙勒克腾山及周缘花岗岩为研究对象,在地质调查的基础上,进行岩石学、锆石U-Pb年龄、岩石地球化学特征研究,探讨其形成时代及构造背景。西准噶尔白杨河上游沙勒克腾山及周缘花岗岩可划分为沙勒克腾、查干恩干、阿拉乔古、坦子不拉克4个序列,各序列岩石组合分别为花岗闪长斑岩、二长花岗斑岩,碱长花岗岩、正长花岗岩,闪长玢岩、闪长岩、石英闪长玢岩,二长花岗斑岩、花岗斑岩。沙勒克腾序列锆石U-Pb年龄为302.1±1.4 Ma(n=26,MSWD=1.5),形成时代为晚石炭世;具有低硅(SiO2=68.07%~70.49%)、贫碱(K2O+Na2O=7.23%~8.18%)、富铝(Al2O3=14.12%~15.69%)、富钛(TiO2含量0.43%~0.48%)、富钙(CaO含量1.71%~3.31%)特征;相对亏损Nb、Ta、Sr、P、Ti,富集Zr、Hf。查干恩干序列锆石U-Pb年龄为296.7±2.1 Ma(n=26,MSWD=0.41),形成时代为早二叠世早期;具有相对高硅(SiO2=74.21%~74.37%),富碱(K2O+Na2O=8.84%~9.19%)、富铝(Al2O3=12.59%~12.85%),低钛(TiO2=0.19%~0.22%)、低钙(CaO=0.64%~1.24%)特征;稀土元素含量低,亏损Sm、Tb、Y、Yb、Lu、Sr,富集Zr、Hf。沙勒克腾序列、查干恩干序列、阿拉乔古序列、坦子不拉克序列分别属于准铝质—弱过铝质高钾钙碱性I型花岗岩、准铝质碱性高分异I型花岗岩、弱过铝质碱性I型花岗岩、S型花岗岩。研究表明,白杨河上游库勒克腾山及周缘花岗岩形成于后碰撞环境挤压应力体制向伸展应力体制转换期,查干恩干序列的侵位标志着区域应力体制进入伸展阶段,进而限定研究区进入伸展应力体制的时限应不晚于296.7±2.1 Ma,为西准噶尔地区晚石炭世—早二叠世后碰撞构造研究提供了新的依据。     

江苏省东海县片麻状碱性花岗岩的成因及年代学研究

江苏省东海地区位于苏鲁造山带的西南缘,区内广泛出露片麻状碱性花岗岩.本文对东海地区磨山、虎山和房山片麻状碱性花岗岩的产状、岩石结构构造、碱性铁镁矿物、元素地球化学和锆石成因及SHRIMP定年等综合研究结果表明：该类岩石具有高硅、富碱、富铁贫镁和含碱性铁镁矿物等特征,说明其为碱性花岗岩;岩体还保留有沉积岩特有的层理（走向北东、倾向南东等一致的产状）,长石等造岩矿物显示在固相条件下结晶的特征,以及继承锫石大部分是不同时代的岩浆或变质锆石碎屑等,均证实其原岩是沉积岩.SHRIMP定年对变质锆石的边部获得两组206Pb-238U加权平均年龄,分别为224.7±9.4 Ma和209.4±2.5～212±3.5Ma,反映了东海片麻状碱性花岗岩成岩时间属于印支期,相当于晚三叠世,变质锆石的核部206Pb-238U年龄变化于866～248 Ma,反映原岩时代不会早于晚古生代.