SHRIMP U-Pb Dating of Zircons of a Dark Eclogite and a Garnet-bearing Gneissic Granitic Rock from Bixiling,Eastern Dabie Area,Anhui Province:Isotope Chronological Evidence of Neoproterozoic UHP Metamorphism

The paper reports SHRIMP U-Pb zircon data of a dark eclogite and a post-eclogite garnet-bearing gneissic granitic rock from the Bixiling area, Yuexi County, Anhui Province, in the eastern Dabie Mountains. The eclogite, which is metamorphosed basic tuff, contains very scarce zircons in omphacite or garnet, but more zircons in quartz. They usually exhibit a double-layered texture, as shown clearly in cathodoluminescence images. Their inner main parts give a 206Pb/238U age of 757±7 Ma, representing the approximate age of the high-pressure (HP)- ultrahigh-pressure (UHP) metamorphic event during which the eclogite was formed. The outer peripheral parts of the zircons, which have been modified by late-stage fluids, give an age of 223±3 Ma. The granitic rock contains more zircons of anatectic origin found mostly in feldspar and quartz and usually also showing a similar composite texture. The inner main parts of the anatectic zircons with oscillatory zoning give a 206Pb/238U age of 727±15 Ma for the approximate age of the emplacement of the granitic rock, and their outer parts, an age of 219±3 Ma for a similar or even the same fluid event. It is thus suggested that the HP-UHP metamorphism of the Bixiling eclogite facies rocks took place during the Neoproterozoic Jinningian, and the Indosinian age values may only represent a late event in the nature of fluid activity.

Geochemistry and petrogenesis of the Paleoproterozoic orthogneisses and granitoids of the Banded Gneissic Complex,central Rajasthan,NW India:Implications for crustal reworking processes

Granitoids and orthogneisses(1.7 Ga)from the northern part of the Banded Gneissic Complex(BGC-II domain),Aravalli Craton(NW India)are geochemically analyzed to understand the geodynamic condition and crustal evolution processes.The samples are metaluminous to peraluminous(molar A/CNK:0.74±2.12),and characterized by Eu-anomaly ranging from 0.17 to 1.06,Fe_(2)O_(3)^(T)/(Fe_(2)O_(3)^(T)+MgO)from 0.8 to 0.9 and high magmatic zircon saturation temperature(>830°C)which are the features suggestive of A-type granite affinity.Tectonic discrimination diagrams classify the samples as post-collisional extensional A2-type granites.Geochemical characteristics along with trace element ratios[(Y/Nb)_(N)=0.15 to 4.33(avg.0.76),(Th/Nb)_(N)=4.63 to 255.47(avg.63.13),(Th/Ta)_(N)=1.37 to 9.84(avg.8.86),(Ce/Pb)_(N)=0.05 to 3.05(avg.1.43)]indicate that the rocks were derived from a plagioclase-rich and garnet-free crustal source under lowpressure conditions.Further,it is also proposed that tonalite-trondhjemite-granodiorite(TTG)rocks which occur dominantly in southern Rajasthan(BGC-I)are precursors and their partial melting led to the generation of the studied A-type granite samples.The studied samples also bear close geochemical similarity with A-type granites of similar age(1.7 Ga)near the Sakhun-Ladera region of northern BGC-II.The studied A-type granites are believed to be coeval to similar aged A-type granites of the Khetri and Alwar sub-basins of the North Delhi Fold Belt(NDFB).They are comparable in age and magmatic history to recorded A-type magmatism in North America and parts of the Chinese craton.The large geographical extents of synchronous A-type granites are proposed to be related to the Columbia Supercontinent assembly(ca.1.7 Ga;postcollisional granites).Thus,based on the studied extensional granites,we surmise that BGC-II was part of the Columbia Supercontinent.

Geochemistry and Petrogenesis of Late Cretaceous–Paleocene Granites from the Tengchong Block, Western Yunnan: Implications for Angle-switching of Subducting Slab

The Cenozoic geological hallmark of Western Yunnan is the characteristic voluminous Late Cretaceous-Eocene granites;however, their geological background and petrogenesis have not been well constrained and elucidated. In this study, we present new zircon U-Pb dating, along with geochemical and Sr-Nd-Hf isotopic data for granites from the Tengchong–Lianghe granitoid belt(as abbreviated to Tengliang belt) and West Yingjiang batholiths from the Tengchong block. The mineralogical and geochemical features of the Tengliang granites and the West Yingjiang batholiths are ascribed to aluminous S-type granites and weak peraluminous I-type, respectively. Zircon U-Pb analyses yielded consistent ages ranging from 67.5 Ma, 68.4 Ma and 66.2 Ma from the Tengliang granitoid belt and 50.4 Ma to 60.8 Ma for three samples from the west Yingjiang batholiths. The Tengliang granites were emplaced during the Late Cretaceous(68-66 Ma) and demonstrate negative ε_(Hf)(t) values(-24 to-4) and initial ~(87)Sr/~(86)Sr ratios of 0.7101–0.7139 and significant negative ε_(Nd)(t) values from-8.91 to-13.2, indicating a Proterozoic sedimentary source or enriched components. The hornblende-bearing I-type granites from West Yingjiang are characterized by lower initial ~(87)Sr/~(86)Sr ratios of 0.7076–0.7106, compared to Tengliang granite and negative whole-rock ε_(Nd)(t) values from-4.0 to-11.9. The early Eocene west Yingjiang gneissic granites show wide ranges of ε_(Hf)(t) values from +7.4 to-8.5 and T_(2DM) of 1.30–0.65 Ga, indicating partial melting of ancient crust with contributions of depleted mantle materials. In combination with the regional background and previous studies, we propose that such a spatio-temporal distribution of the Tengchong granitoid belt might be related to the rollback or angleswitching of the Neo-Tethyan subducting slab. This study sheds new light on the evolutionary history of the Tengchong block.

U-Pb zircon age of the foliated garnet-bearing granites in western Dabie Mountains,Central China

U-Pb zircon dating on two foliated garnet-bearing granite samples in the western Dabie ultra-high-pressure (UHP) metamorphic unit yields concordant ages of (234±4) Ma and (227±5) Ma, respectively. These ages, following the UHP peak metamorphism, represent the magma emplacement ages for the foliated garnet-bearing granites. This, for the first time, shows that there are the Triassic granites in the Dabie Mountains. The foliated garnet-bearing granites resemble A-type granite in geochemical characteristics, indicating that they were formed in exten-sional geodynamic setting. The magma formation reflects a reheating event in the Dabie orogenic belt and it enhances the transfer of tectonic regime from collision into extension and promotes the rapid exhumation into lower crust for the UHP metamorphic rocks.

Petrogenesis of the Borjuri diorite pluton in the Mikir Massif of Northeast India:implications for post-collisional intermediate magmatism during the Pan-African orogeny

The Assam-Meghalaya Gneissic Complex(AMGC)of northeast India contains numerous Pan-African granitic bodies that have been attributed to post-collisional rift-related magmatism.The present study is concerned with the first appraisal of intermediate magmatism(diorite,monzonite,and monzodiorite)found in the Borjuri Pluton of Mikir Massif,which is the eastern extension of AMGC.The diorites are strongly metaluminous and exhibit enriched LREE([La/Yb]N=1.63–7.37)with respect to HREE([Gd/Yb]N=1.95–2.27).The studied rocks do not show any mineralogical or textural indication of metamorphism.Tectonic discrimination diagrams indicate that these rocks originated in a within-plate tectonic setting.The lower Mg#(33.49–38.69),low Cr(below detection limit),and Ni(27–41 ppm)contents along with elemental ratios such as Rb/Sr(0.32–0.95),La/Nb(0.49–4.21),and Nb/Ce(0.11–0.64)suggest a crustal source for the diorites.Discrimination diagrams coupled with elemental ratios suggest that these rocks originated due to partial melting of mafic components in the crust with possible contribution from mantle materials.The P–T conditions of diorite emplacement(7.4 kbar,688℃)were calculated using the amphibole-plagioclase geothermobarometer.Geochemical and geochronological data of the Pan-African felsic plutons reported from the AMGC indicate that these rocks were emplaced in a post-collisional extensional regime.The Borjuri Pluton is in close proximity with the Kathalguri Pluton,which has been reported as a product of PanAfrican magmatism.In view of the numerous extensional Pan-African felsic magmatism reported from the AMGC and based on the close vicinity of the Borjuri diorites with the Kathalguri granites,we speculate that the Borjuri diorites are products of the Pan-African post-collisional magmatism.

阿尔泰造山带阿巴宫花岗岩体锆石SHRIMP年龄及其地质意义

阿尔泰造山带分布着大量花岗岩。本次研究的出露于阿巴宫一带的两个片麻状花岗岩体,其锆石SHRIMP U-Pb年龄分别为(462.5±3.6)Ma和(457.8±3.1)Ma,在误差范围内年龄一致,表明这两个岩体是同期岩浆作用的产物。较老的年龄说明该岩体原被划分为华力西中晚期侵入岩体的认识并不正确,该岩基可能是由不同时代的侵入体组成。阿巴宫2号岩体呈岩株状出露于下泥盆统康布铁堡组火山岩地层中,二者之间没有显示侵入接触和断层接触关系,表明火山岩地层可能为岩体的不整合沉积盖层,后经剥蚀和构造作用使岩体局部出露地表。

北京云蒙山片麻状花岗岩锆石SHRIMP定年及其地质意义

应用锆石SHRIMP定年方法对云蒙山片麻状花岗岩进行年代学研究 ,得到 4组年龄 :14 4± 4Ma、16 0～ 16 3Ma、193～ 2 18Ma和 2 4 16Ma。其中 14 4± 4Ma代表了云蒙山岩体的侵位时间 ,16 0～ 16 3Ma和 193～ 2 18Ma两组年龄可能是岩浆侵位过程中捕虏锆石的年龄。 2 4 16Ma与Davis等的锆石U_Pb法上交点年龄 (190 0～ 2 4 0 0Ma)一致 ,可能反映了原岩的时代 ,说明该花岗岩来源于晚太古代片麻岩的局部熔融或者是岩浆侵位过程中捕获了晚太古代的锆石。

滇西腾冲地块片麻状花岗岩的锆石U-Pb年龄

报道了滇西腾冲地块片麻状花岗岩的锆石U-Pb年龄。区域地质和岩相特征表明,片麻状花岗岩为岩浆成因,并经历了后期深熔作用。24颗锆石测得24个年龄数据中,1颗继承锆石的206Pb/238U年龄为537.5±5.6Ma,2颗岩浆锆石的206Pb/238U平均年龄为456Ma,3颗深熔锆石的206Pb/238U平均年龄为117Ma,18颗深熔锆石的Robust(Median)206Pb/238U年龄为66.2-+47Ma。上述结果表明,花岗岩形成于早古生代,而后在白垩纪发生深熔作用,可能是特提斯演化过程中的岩浆响应。

阿尔金南缘塔特勒克布拉克复式花岗质岩体东段片麻状花岗岩的地球化学特征、锆石U-Pb定年及其地质意义

出露于阿尔金构造带南缘北部塔特勒克布拉克复式花岗质岩体东段的片麻状花岗岩,SiO2为71.88%~73.92%,Al2O3为13.39%~14.14%,K2O＋Na2O为8.18%~8.85%,K2O/Na2O=1.54~2.33,A/CNK介于1.02~1.09之间,属高钾钙碱性系列的弱过铝质岩石。该岩石富集大离子亲石元素（LILE）,亏损Nb、Ta、P、Ti等高场强元素（HFSE）;∑REE较高且变化范围大（∑REE=98.57×10-6~579.1×10-6,平均338.8×10-6）,具有明显的负Eu异常（δEu=0.22~0.71,平均0.34）,LREE相对富集,轻重稀土分馏明显。Nb/Ta=11.78~15.83、Zr/Hf=34.94~36.82及Th/U=8.4~12.7,结合源岩判别图解,推断其源区物质主要来源于上地壳的变泥砂质沉积岩类。微量元素及稀土元素特征暗示原岩部分熔融残留相的矿物组合可能为石榴石＋斜长石＋金红石,全岩Zr饱和温度计计算结果显示部分熔融温度〉800℃,推断该岩石形成于麻粒岩相条件下云母和角闪石脱水诱发的部分熔融。该岩石LA-ICP-MS锆石U-Pb原位定年结果为：761±54Ma（上交点年龄）,451±1.7Ma（核部）和411.3±1.8Ma（边部）,锆石核部Th/U平均为0.64,边部Th/U平均为0.05。结合区内超高压榴辉岩的原岩形成时代、峰期变质与退变质时代分别为约750Ma、500Ma与450Ma的研究资料（Liu et al.,2012）综合分析,塔特勒克布拉克片麻状花岗岩的原岩时代为782.3±6.9Ma,可能与罗迪尼亚超大陆裂解事件有关;花岗岩结晶年龄为451±1.7Ma,形成于俯冲碰撞造山后抬升阶段,对应于区内深俯冲陆壳的折返时代,此时超高压变质岩石发生了麻粒岩相的退变质作用,随后411.3±1.8Ma又受到另一期地质事件的改造。

中天山东段澄江期片麻状花岗岩特征和成因——以天湖东岩体为例

本文以天湖东岩体为例,探讨中天山澄江期片麻状花岗岩的特征及其形成机制。岩相学、岩石化学、稀土元素、微量元素和同位素研究表明,天湖东岩体是岛弧钙碱性火山—沉积岩系经原地改造的产物,其Rb-Sr等时线年龄为707.7±4.9Ma。花岗岩形成作用的动力、热力来源可能与向塔里木大陆俯冲的天山岩石圈的拆沉有关。在造山过程晚期-期后的挤压向拉张转变过程中,拆沉导致的底侵和内侵引发了地壳岩石的脱水和熔融,所产生的熔体-流体上升并将较浅层次的变质岩改造为片麻状花岗岩。变质峰期以后形成的韧性剪切带为熔体-流体的运移提供了通道。

西藏南部康马岩体岩石类型及其同位素测年

康马岩体位于西藏南部康马县城北侧 ,其内部可划分出 5种不同的岩石类型 :(1)片麻状黑云二长花岗岩 ;(2 )眼球状黑云二长花岗岩 ;(3)片麻状二云母二长花岗岩 ;(4 )弱片麻状细粒黑云二长花岗岩 ;(5 )变质的暗色辉长辉绿岩。前两类为康马岩体的主体岩石类型 ,侵位时代为加里东早期 (约4 78～ 4 6 1Ma) ;其余均呈脉状产出 ,其中片麻状二云母二长花岗岩与康马岩体主体岩石类型属于同期产物 ,弱片麻状细粒黑云二长花岗岩形成于海西早期 ((339.0± 1.2 )Ma) ,暗色辉长辉绿岩的形成时代目前还不十分清楚。不同类型的岩石其Ar Ar冷却年龄约为 18～ 14Ma。

大别山碧溪岭深色榴辉岩和片麻状花岗质岩石SHRIMP分析——晋宁期高压—超高压变质作用的同位素年代学依据

本文报道大别山安徽省岳西县碧溪岭深色榴辉岩和片麻状花岗质岩石的锆石SHRIMP分析结果。深色榴辉岩的原岩为基性凝灰岩,锆石在岩石中主要产于石英、石榴子石和绿辉石内,为高压一超高压变质作用的产物,内部主体形成年龄约为757±7Ma,代表高压—超高压榴辉岩相变质事件的时代;经受了后期流体改造的锆石边部年龄为223±3Ma。在片麻状花岗质岩石中,发育韵律环带结构的深熔锆石形成于约727±15Ma,受后期流体改造的锆石边部年龄为219±3Ma。综合野外地质关系、区域地质背景和锆石的岩相学特征,笔者提出了碧溪岭榴辉岩相岩石高压—超高压变质作用发生于晋宁期,印支期仅代表一次后期流体改造事件的观点。

内蒙古北山造山带变辉绿岩和片麻状花岗岩锆石U-Pb年龄、Hf同位素组成及地质意义

北山造山带位于西伯利亚、哈萨克斯坦和塔里木三大板块交汇部位。通过对内蒙古北山造山带变辉绿岩和片麻状花岗岩的岩石学、岩石地球化学、锆石阴极发光结构、锆石SHRIMP U-Pb年龄、LA-MC-ICPMS锆石Lu-Hf同位素的多方面系统研究,并结合前人的研究成果,取得了如下主要认识:变辉绿岩具有略向右倾的稀土元素分布型式,Eu异常不明显,并亏损Ta、Nb等高场强元素;片麻状花岗岩稀土元素分布型式明显右倾,具Eu负异常与Nb、Ta、Zr、Hf、HREE亏损。变辉绿岩中的锆石主体结晶于114.8Ma±3.3Ma,为燕山期岩脉;片麻状花岗岩中锆石主体的结晶年龄为423Ma±4Ma,为加里东晚期的岩体,从而更正了过去对其年龄为古元古代或石炭纪的认识。变辉绿岩在岩浆侵位过程中同化混染了地壳组分,捕获了地壳中从中太古代到早白垩世的继承锆石。研究区从中太古代晚期至古元古代晚期,至少曾发生过5期壳幔分异事件,导致了地壳增生,并形成了该区古老的地壳基底。

华北地台北缘集宁地区古元古代片麻状石榴花岗岩的深熔成因及地质意义

本文对华北克拉通北缘集宁地区空间上密切共生的片麻状石榴花岗岩和孔兹岩系富铝片麻岩的岩相学、地球化学及年代学特征进行了对比研究。SHRIMP锆石U-Pb定年方面,在富铝片麻岩中获得了1910±10Ma和1839±13Ma变质锆石年龄,在片麻状石榴花岗岩中获得了1919±17Ma的变质重结晶锆石年龄。在石榴花岗岩的石榴石包裹体中识别出与富铝片麻岩相对应的进变质阶段(M1)和峰期阶段(M2)的矿物组合,由此确认富铝片麻岩的变质作用和导致石榴花岗岩形成的深熔作用是同一构造热事件的产物。通过对二者变质作用演化及特征变质矿物的对比,认为深熔作用主要发生在峰期后等温降压阶段(M3),石榴花岗岩中的石榴石为深熔作用过程中的残留矿物相或转熔矿物相,而石榴花岗岩则是混合有大量残留矿物相的熔体结晶的产物。对片麻状石榴花岗岩和富铝片麻岩的地球化学组成特征进行了对比分析,片麻状石榴花岗岩既有一定的继承性,又有十分明显的变异性。变异性表现为:1)石榴花岗岩主量和微量元素含量分布极不均匀,微量元素含量普遍低于源岩(Cs、Rb、Th、U、Nb、Ta、Zr、Hf等);2)大离子亲石元素Cs和生热元素U、Th亏损明显,Sr相对富集;3)高场强元素Nb、Ta、P、Ti的明显亏损;4)铕异常变化大,存在铕富集型、铕平坦型和铕亏损型共存的稀土配分曲线的岩石,这是深熔成因石榴花岗岩最突出的表现,也可能是原地-半原地深熔花岗岩的主要地球化学标志。综合区域上的地质资料,认为深熔作用与碰撞后伸展构造背景下基性岩浆底侵事件有关。

大别山碧溪岭未经历超高压变质的片麻状花岗岩

关于大别山超高压变质带中与榴辉岩密切伴生的花岗片麻岩是否也经历了超高压变质作用一直是个争议的问题。笔者最近在碧溪岭片麻状花岗岩中首次发现了可靠的岩相学和矿物学证据 ,证明该岩石只遭受到绿片岩相变质作用的改造 ,而未曾记录超高压变质矿物组合。根据片麻状花岗岩与围岩接触部位强烈糜棱岩化的发育 ,认为超高压岩石在抬升到地壳范围时与以花岗质岩石为主构成的扬子陆壳发生了大规模的构造并置。

华北克拉通胶—辽—吉带古元古代条痕状花岗岩成因及其构造意义

华北克拉通胶一辽一吉带内发育大量古元古代条痕状花岗岩，岩性主要为含角闪石或磁铁矿的二长花岗岩，以暗色矿物定向排列为显著识别特征。基于系统的野外调查、岩石学、地质年代学和地球化学研究，对条痕状花岗岩成因和其形成的构造环境提出了不同的模式。LA-ICp-MS锆石U-Pb年龄显示这些条痕状花岗岩形成于2173～2205Ma，基性岩脉侵位于2159Ma，后期的角闪岩相变质发生于约1907Ma。条痕状花岗岩中发育基性暗色微粒包体，也发育钙质角闪石和镁铁质角闪石，副矿物为锆石、榍石和磁铁矿。主量和微量元素分析结果显示：条痕状花岗岩属于钙碱性岩浆系列，A／CNK值为0．9～1．2（多数小于1．1），A／NK值为0．9～1．4，SiO2质量分数为68．3％～76．9％，岩石具有低TiO2质量分数（小于0．3％）；岩石富集大离子亲石元素（如K、Rb、Sr、Cs），亏损部分高场强元素（如Nb和Ti）。这些特征都指示条痕状花岗岩可能是形成于俯冲带环境下的I型花岗岩。部分岩体（如大房身岩体）具有类似A型花岗岩的特征，这是岩浆体系富硼而引起的高分异所致的，岩体高SiO2质量分数（76．7％～77．1％）也是高分异的表现。硼的加入会导致岩浆体系固相线温度降低，使得岩浆具有更为充分的时间进行演化。全岩εNd（t）值（一8．6～1．5）和锆石εHf（t）值（一1．3～5．6）都具有非常宽的变化范围，结合基性暗色微粒包体的岩相学特征以及斜长石的环带，说明条痕状花岗岩形成于下地壳熔融的酸性岩浆和富集地幔熔融的基性岩浆的混合。花岗岩的弧岩浆属性及其老于2500Ma的太古代继承锆石指示胶一辽一吉带在古元古代可能发生了北向俯冲。

云开隆起区钾长球斑片麻状黑云母花岗岩锆石特征研究——形貌、成分以及U-Pb同位素年龄

云开隆起区核部广泛出露的钾长球斑片麻状黑云母花岗岩中存在形貌、成分不同的3类锆石,这些锆石的单颗粒U—Pb同位素年龄明显不同,说明此类陆壳改造型花岗岩源岩成因复杂,源岩时代老于1364Ma,岩体形成时代为463～508Ma。

蚌埠老山含石榴子石片麻状花岗岩的锆石SHRIMP U-Pb年龄及其对华南俯冲陆壳再循环的意义

对华北板块东南缘蚌埠隆起区内凤阳县老山含石榴子石片麻状花岗岩进行了锆石SHRIMP U—Pb定年。阴极发光图像显示所研究样品中锆石通常具有核-幔-边结构。其中，核部年龄为新元古代（一个谐和年龄为673Ma±12Ma）、幔部年龄为三叠纪和具有岩浆结晶环带的边部年龄为159Ma±2Ma。锆石边部明显具有岩浆结晶环带以及高的Th／U比值（一般为0．14～0．45），其年龄代表该岩石的形成时代；幔部具有低的Th／U比值（一般为0．02～0．15），为变质成因；而核部常具有岩浆结晶环带，有时因重结晶等原因而模糊，其年龄应代表源区岩石的形成时代。由于继承锆石的形成时代为新元古代并经过印支期变质作用，这些年龄记录与大别山超高压岩石一致。因此，证明老山含石榴子石片麻状花岗岩为159Ma±2Ma，其为经历了三叠纪多期变质作用改造的华南俯；中陆壳物质发生部分熔融而形成即由华南三叠纪俯冲陆壳物质再循环的结果。而华北板块通东南缘深部存在华南俯冲陆壳岩石，进一步证实在中生代华南板块至少已俯冲至蚌埠地区。

江苏东海片麻状碱性花岗岩地质地球化学及其年代学研究--以驼峰、牛山两地为例

本文通过对东海县驼峰和牛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,得出以下结论：①驼峰和牛山片麻状碱性花岗岩,在造岩矿物和岩石化学成分等方面具有与岩浆成因的钠质碱性花岗岩一致的特点：高硅（SiO2=74.25%~77.08%）、富碱（Na2O＋K2O=7.7%~9.31%）、高钠（Na2O〉K2O）、低钙（CaO=0.18%~0.39%）和含霓石等碱性铁镁矿物;②东海县驼峰和牛山片麻状碱性花岗岩片理产状：走向北东、倾向南东、倾角30°~35°。造岩矿物：斜长石聚片双晶常垂直晶体延长方向等,显示在固相条件下结晶的特征,以及继承锆石大部分是不同时代的岩浆碎屑锆石。上述特征均显示其原岩应是沉积岩;③锆石SHRIMP定年结果表明,驼峰和牛山片麻状碱性花岗岩成岩年龄为晚三叠世,其原岩时代不会早于古生代。

新元古代Rodinia超大陆裂解事件在扬子北东缘的最初响应:东海片麻状碱性花岗岩的锆石U-Pb年代学及Nd同位素制约

东海片麻状碱性花岗岩是扬子克拉通北东缘最具代表性的含碱性铁镁矿物的过碱性 A 型花岗岩。高精度的LA-ICP-MS 锆石 U-Pb 定年结果表明,这些岩石形成于新元古代中期770Ma 左右,并在约250Ma 时经历超高压变质作用,之后又于210Ma 左右遭受了大规模的角闪岩相退变质作用的改造。Nd 同位素示踪表明东海片麻状碱性花岗岩具有相对偏高的ε_(Nd)(t)值(+1.4～-5.91),指示其应是成熟度较低的下地壳部分熔融的产物,但不排除部分岩体在成岩过程中可能有少量幔源物质卷入。综合 Rodinia 超大陆古地理复原资料,推测扬子板块北东缘自新元古早期以来一直为接受大洋板块俯冲的活动大陆边缘。形成于新元古代中期的东海片麻状碱性花岗岩属 A_2型花岗岩,为弧后拉张背景下岩浆活动的产物,该类岩石的产出代表了 Rodinia 超大陆裂解事件在该地区的最初响应。