东天山印支早期尾亚石英正长岩：成岩作用及成岩意义

前人对东天古生代的构造-热事件作了大量研究,得出了许多方面的重要认识,然对东天山印支期的构造演化目前仍不清楚。本文以印支早期的尾亚石英正长岩为例,通过其岩相学及地球化学特征研究,来探讨该岩体的成岩机制及成岩意义。尾亚石英正长岩主要组成矿物为石英(5%～10%)、斜长石(15%～20%,An16～18)和钾长石(60%～65%),含少量铁质黑云母(5%)和钙质角闪石(≈5%),不舍橄榄石和辉石。该岩石SiO_2含量为62．67%～66．22%,Na_2O+K_2O(10．25%～11．00%)和TiO_2(0．45%～0．70%)含量以及K_2O/Na_2O(0．85～1．60)、FeO~T/Mgo(5．7～15．86)和CaO/Na_2O(0．37～0．68)比值高,而Mgo(0．34%～0．43%)含量以及Al_2O_3/TiO_2(23．77～38．27)比值低,偏铝(ASI 0．89～0．98),富集Rb、Ba和Pb等大离子亲石元素(LILE)和U、Th、Zr和Hf等高场强元素(HFSE),亏损Sr等大离子亲石元素及Nb和Ti等高场强元素。ΣLREE,ΣHREE和ΣREE分别为76．00×10~(-6)～139．54×10~(-6),13．54×10~(-6)～22．67×10~(-6)和96．57×10~(-6)～153．08×10~(-6), (La/sm)_N,(Gd/Yb)_N,(La/Yb)_N分别为1．86～4．78,0．73～1．64和2．10～9．64,δEu为0．94～2．83,具有稀土总量较低、轻稀土比重稀土分馏大、轻、重稀土分馏不明显以及正Eu异常(0．94～2．83)的特征。岩相学及地球化学特征研究表明,尾亚石英正长岩不是幔源岩浆分异的产物,而是直接来自于壳源。通过对比分析其它正长岩地球化学特征并结合前人实验岩石学研究,得出了尾亚石英正长岩是在950℃～1100℃、15～20 kbar条件下,由陆内长英质麻粒岩在加厚陆壳底部发生增温和减压熔融的产物的认识。这样的成岩条件可能与特提斯洋于海西晚期—印支早期向北俯冲所引发的天山地区强烈的陆内A-型俯冲及随后的伸展作用有关。本文结果表明,东天山地区在二叠纪晚期到三叠纪早期可能发生了从受古亚洲洋俯冲-碰撞体系影响为主转变为受古特提斯洋构造域影响为主的重大事件。     

内蒙古迪彦庙蛇绿岩带早石炭世闪长岩成因与古亚洲洋洋内俯冲作用

由于缺少对SSZ型蛇绿岩和洋内弧火成岩的系统研究,制约了古亚洲洋东段古生代洋内俯冲过程的进一步认识。本文报道了内蒙古迪彦庙SSZ型蛇绿岩带北部新发现的巴嘎哈尔早石炭世闪长岩。LA-ICP-MS锆石U-Pb定年显示,巴嘎哈尔闪长岩的侵位年龄为324.2±1.8 Ma,其形成时代为早石炭世末期。巴嘎哈尔闪长岩SiO₂含量为57.71%～61.24%;高铝(Al₂O₃含量为15.58%～16.68%);具有相对富钠(Na₂O含量为3.29%～4.15%)、低钾(K₂O含量为1.05%～1.69%)的特征,Na₂O/K₂O比值为2.18～3.95;MgO含量较高(3.30%～4.23%),Mg^#为47.20～51.82;贫TiO₂(0.65%～0.76%)和P₂O₅(0.14%～0.15%);稀土元素总量(ΣREE为85.19×10^-6     

中天山地块尾亚地区早古生代构造格架——来自野外地质和锆石U-Pb年代学的证据

野外地质调查发现尾亚地区分布的主体为岛弧杂岩,被更晚的造山后花岗岩如钾长花岗岩等侵入改造。尾亚地区的岛弧杂岩受造山作用的改造,局部发生了一定程度的变质变形作用。这套变质岩分布局限,变质程度较低且主要为绿片岩相,劈理化强。2个样品YM01、YM02分别为黑云斜长变粒岩、角闪斜长变粒岩,其原岩分别为中酸性火山碎屑岩及闪长岩。2个样品的锆石年龄记录了中天山地块北侧的岛弧岩浆事件,表明古亚洲洋(北天山洋)从奥陶纪已经开始,一直持续到泥盆纪由北向南持续俯冲。俯冲过程在晚寒武世开始,暗示中天山地块北侧在当时已经发生了构造转换,作为一个微陆块由被动大陆边缘转变为主动大陆边缘。另外,岛弧杂岩体中没有前寒武纪年龄记录的特征,表明区域内的前寒武纪基底物质分布有限,而变质变形强烈的部分多为造山过程的产物,是在造山带后期改造形成的。     

内蒙古梅劳特乌拉蛇绿岩中早二叠世高镁闪长岩的发现及洋内俯冲作用

内蒙古梅劳特乌拉SSZ型蛇绿岩中,新发现了早二叠世巴嘎哈尔高镁闪长岩。该闪长岩位于贺根山缝合带内。锆石LA-ICP-MS U-Pb测年表明,巴嘎哈尔闪长岩的侵位年龄为(282±2)Ma,其形成时代为早二叠世。该岩石高镁,MgO含量5.00%~10.94%,Mg#值54~74;SiO_2含量53.52%~58.64%,Al_2O_3含量13.48%~14.98%,Na_2O含量1.36%~3.59%;贫钾,K_2O含量0.48%~1.61%;贫TiO_2(0.28%~0.76%)和P_2O_5(0.080%~0.160%);富Cr(75.00×10~(-6)~555.90×10~(-6))和Ni(26.20×10~(-6)~228.40×10~(-6));富集K、Rb和Sr等大离子亲石元素,亏损Nb、Ta、Zr、Ti和P等高场强元素;稀土元素总量较低(25.16×10~(-6)~83.57×10~(-6)),稀土曲线为略右倾的平坦型。该岩石具有高镁闪长岩的地球化学特征,与赞岐岩相类似,属于俯冲带岛弧型岩浆岩。结合其与梅劳特乌拉SSZ型蛇绿岩中玻安岩和埃达克岩的岩石组合时空分布特征,表明古亚洲洋贺根山洋盆在早二叠世可能仍然处于大洋俯冲消减状态。该高镁闪长岩可能为俯冲洋壳+俯冲深积物来源的硅质熔体交代上覆地幔楔使其部分熔融的产物。     

识别洋陆转换的岩石学思路——洋内弧与初始俯冲的识别

阐述了洋陆转化形成的洋内弧与初始弧的岩石组合序列及其地球化学特征,提出岩浆弧是由洋陆转化以及底侵的壳幔转化共同作用形成的认识,前弧环境是洋陆转化形成初生大陆的场所,由特征的类似洋中脊的洋内弧前弧玄武岩类构成。大陆的形成过程如下:从地幔中生长出洋壳,从洋壳中的洋陆转化生长出不成熟的弧陆壳,最后从弧陆壳底侵的壳幔转化中长出成熟的陆壳。这样,地壳的生长和形成主要通过岩浆增生作用来实现。     

Early Indosinian Weiya Gabbro in Eastern Tianshan, China： Elemental and Sr-Nd-O Isotopic Geochemistry, and Its Tectonic Implications

The Weiya gabbro in eastern Tianshan was formed during the early Indosinian. This rock, with low ratios of Ce/Pb （5.74-10.16）, is notably characterized by enrichment in large ion lithophile elements （LILE）, such as Rb, K, Ba and Pb, and in high field strength elements （HSFE）, such as U and Th, but depletion in Nb and Ta. All samples of the Weiya gabbro display similar chondrite-normalized patterns with moderate enrichment in LREE （72.58-135.61ppm）, moderate depletion in HREE （15.26-25.31ppm） and mild fractionation between LREE and HREE （L/ H=4.09-5.98）. The average initial Sr value of the rock is 0.7069, and δ18O values of the rock range from 5.67‰-8.04‰. In terms of Nd isotope ratios, the Weiya gabbro is characterized by positive eNd（t） values （0.52-0.76）. All these characteristics indicate that the source region of the Weiya gabbro was metasomatized by fluids released from subducted young continental crust, with limited crustal contamination during magma ascent and emplacement. Continental （A-type） subduction was induced by northward subduction of the Paleo-Tethyan oceanic plate during the latest Permian to Triassic. From this point of view, it is supposed that tectonic conversion from the Paleo-Asian to the Paleo-Tethys regime occurred during the latest Permian or earliest Triassic.     

Zircon SHRIMP Dating for the Weiya Pluton, Eastern Tianshan: Its Geological Implications

1 Introduction Voluminous literature has resulted from study of the geology and tectonic evironments of post-orogenic granitoids in eastern Tianshan (He et al., 1995; Han et al., 1997; Gu et al., 1999, 2001; Li et al., 2003; Chen et al., 2004). Previous contributions (Hu et al., 1997; Li et al., 2003) indicate that major events of continental collision in northern Xinjiang occurred around 300 Ma. Then, the tectonics converted from compression to extension, resulting in the uplifting of th…     

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.     

关于东天山花岗岩与陆壳垂向增生的若干认识

东天山海西期主碰撞以后形成的花岗岩可分为三个阶段：挤压．伸展转折阶段（310～285Ma）、碰撞后伸展阶段（285-250Ma）和板内阶段（250-208Ma）。这三个阶段在岩石圈厚度、等温线形态和动力学状态等方面的差异，造成了不同阶段花岗岩岩体形态、岩石组合、岩相学和地球化学等方面的差异。除了底侵以外，幔源岩浆的内侵可以造成地壳不同层次岩石的部分熔融，也是花岗质岩浆生成的重要机制和地壳垂向增生的重要方式。东天山的片麻状花岗岩有一部分是变质交代成因的，这类花岗岩的形成与碰撞后幔源岩浆的底侵和内侵有关。变质峰期后韧性剪切带中构造细粒化的岩石是形成片麻状花岗岩的最有利部位。虽然这类花岗岩多数定位于地壳较深层次，但在内侵热量的影响下也可以定位于较浅层次。康古尔韧性剪切带的形成除了构造动力作用以外，还与地壳垂向增生，尤其是内侵有着密切联系，是构造动力、岩浆活动、变质作用和流体运移等复杂反馈的结果。博格达造山带碰撞前和碰撞后岩浆岩均具有正的εNd（t）值，表明该造山带地幔早在碰撞前就已亏损，而碰撞后的地幔则继承了碰撞前地幔的亏损特征。东天山在印支期有一次重要的地壳垂向增生事件，其岩浆活动和成矿作用与古特提期洋的俯冲和随后的碰撞密切有关，因此是东天山从中亚构造体制向特提斯体制转换的产物。     

吐哈盆地南缘彩霞山东石英闪长岩岩株锆石SHRIMP U-Pb测年

彩霞山东石英闪长岩岩株的锆石SHRIMPU-Pb年龄为267.8Ma±1.6Ma,结合克孜尔塔格岩体的同位素年龄综合分析,此年龄值介于王瑜等所研究剪切作用控制的克孜尔塔格岩体右行剪切年龄276～254Ma之间,表明该石英闪长岩岩株的侵位年龄比克孜尔塔格复式岩体晚,为中二叠世早期。     

Multistage metamorphism of the Huangtuling granulite, Northern Dabie Orogen, eastern China: implications for the tectonometamorphic evolution of subducted lower continental crust

Granulites from Huangtuling in the North Dabie metamorphic core complex in eastern China preserve rare mineralogical and mineral chemical evidence for multistage metamorphism related to Palaeoproterozoic metamorphic processes, Triassic continental subduction‐collision and Cretaceous collapse of the Dabie Orogen. Six stages of metamorphism are resolved, based on detailed mineralogical and petrological studies: (I) amphibolite facies (6.3–7.0 kbar, 520–550 °C); (II) high‐pressure/high‐temperature granulite facies (12–15.5 kbar, 920–980 °C); (III) cooling and decompression (4.8–6.0 kbar, 630–700 °C); (IV) medium‐pressure granulite facies (7.7–9.0 kbar, 690–790 °C); (V) low‐pressure/high‐temperature granulite facies (4.0–4.7 kbar, 860–920 °C); (VI) retrograde greenschist facies overprint (1–2 kbar, 340–370 °C). The P–T history derived in this study and existing geochronological data indicate that the Huangtuling granulite records two cycles of orogenic crustal thickening events. The earlier three stages of metamorphism define a clockwise P–T path, implying crustal thickening and thinning events, possibly related to the assembly and breakup of the Columbia Supercontinent at c. 2000 Ma. Stage IV metamorphism indicates another crustal thickening event, which is attributed to Triassic subduction/collision between the Yangtze and Sino‐Korean Cratons. The dry lower crustal granulite persisted metastably during the Triassic subduction/collision because of the lack of hydrous fluid and deformation. Stage V metamorphism records the Cretaceous collapse of the Dabie Orogen, possibly due to asthenosphere upwelling or removal of the lithospheric mantle resulting in heating of the granulite and partial melting of the North Dabie metamorphic core complex. Comparison of the Huangtuling granulite in North Dabie and the high‐pressure–ultrahigh‐pressure metamorphic rocks in South Dabie indicates that the subducted upper (South Dabie) and lower (North Dabie) continental crusts underwent contrasting tectonometamorphic evolution during continental subduction‐collision and orogenic collapse.     

东天山哈尔里克山区二叠纪高钾钙碱性花岗岩成因及地质意义

本文从岩石学、地球化学和同位素等方面讨论东天山克拉麦里-哈尔里克岛弧东段八大石和小铺东两个岩体的特征和成因。八大石岩体和小铺东岩体主要为二长花岗岩,SiO₂含量分别为61.92%~74.40%和69.17%~74.92%,K₂O+Na₂O的含量分别为6.50%~8.32%和7.74%~8.14%,绝大部分岩石具有高钾钙碱性花岗岩特征;∑REE分别为105×10^-6~210×10^-6和100×10^-6~172×10^-6,(La/Yb)_CN分别为4.1~8.9和9.1~15.3,配分模式右倾,δEu分别为0.40~0.93和0.59~0.80,为中-低负异常;两个岩体均富Rb、Ba等大离子亲石元素和Th、U、Hf、Zr等高场强元素,而贫Ta、Nb、Sr、Ti等。通过LA-ICP-MS分析得到八大石岩体中锆石²⁰⁶Pb/²³⁸U加权平均年龄为298±2Ma,表明岩体形成于早二叠纪,计算得到八大石和小铺东岩体的模式年龄t_DM分别为944Ma和648Ma;八大石和小铺东岩体的ε_Nd(t)u分别为+3.06和+4.47,(⁸⁷Sr/⁸⁶Sr) _i分别为0.70475和0.70384,表现出高ε_Nd(t)u 低(⁸⁷Sr/⁸⁶Sr) _i。综上所述,认为哈尔里克高钾钙碱性花岗岩可能为碰撞后挤压-伸展转折阶段的产物,主要由来自新生地壳的中钾钙碱性岩浆经过结晶分异作用而成。     

中天山东段天湖花岗岩岩石学、地球化学及其成因

天湖花岗岩体由六个侵入体组成,按岩石谱系划分为三个岩相单元,归并为一个超单元,其为同一次岩浆熔融事件过程中曾经发生过三次岩浆侵入活动而形成的圆形深成岩体。LA-ICP-MS锆石U-Pb定年结果表明,天湖岩体侵位于早三叠世(241~243Ma),为印支早期的产物。来自岩体的锆石原位Hf同位素测定结果表明,ε_(Hf)(t)值变化于-2.6~5.5之间,二阶段Hf模式年龄(t_(DM2))变化于941~1435Ma之间,表明其岩浆源区均为来源于亏损地慢的新生地壳岩石。该岩体总体富硅(SiO_2为69.14%~76.62%),富碱(K_2O+Na_2O为7.39%~8.83%),而Ti,Ca,Fe和Mg含量较低,A/CNK为0.95~1.04,(K_2O+Na_2O)/Al_2O_3为0.69~0.82,属于钙碱性花岗岩。总体上富集大离子亲石元素(LILE)K、Rb和高场强元素(HFSE)Ba、Th,但贫Ta、Nb、Ce、Hf、Zr、Sm、Y及Yb。稀土元素的球粒陨石标准化配分曲线呈右倾型,轻重稀土分馏明显,重稀土分异不显著,而轻稀土分异明显,铕呈现弱负异常(δEu=0.24~0.78)。岩石学及地球化学特征研究表明,天湖岩体形成与幔源岩浆的底侵和内侵使得中-新元古代新生的地壳物质发生部分熔融有关。岩浆侵入活动的动力来源于海西晚期至印支早期古特提斯洋向北俯冲和碰撞及随后的板内伸展作用。     

新疆西天山科克赛岩体年代学、地球化学及地质意义

新疆西天山科克赛岩体位于赛里木地块北部,岩石类型为二长花岗斑岩,属高钾钙碱性系列.元素地球化学组成表明,该二长花岗斑岩主量元素SiO2 、Al2O3和MgO含量分别为70.56％ ～71.46％、15.27％ ～16.01％和0.77％ ～ 1.02％;轻稀土元素富集,(La/Yb)N为14.99 ～23.69,重稀土元素亏损,δEu值显示弱负异常或正异常(0.88 ～1.21);微量元素Sr、Y和Yb含量分别为363×10-6 ～577×10-6、7.40×10-6～8.50×10-6和0.60×10-6 ～0.72×10-6.上述特征显示二长花岗斑岩具有埃达克岩的地球化学特征.同位素地球化学特征表明该二长花岗斑岩具有较低的锶初始值(ISr=0.705078 ～0.705572)、正的εNd(t)值(2.82 ～3.56)和相对年轻的两阶段Nd模式年龄(774 ～ 835 Ma).锆石LA-MC-ICP-MS U-Pb年龄为301.9±1.8Ma,表明岩体形成于晚石炭世末期.元素地球化学和同位素地球化学特征显示科克赛岩体为新元古代增生的地壳物质部分熔融的产物,形成于晚石炭世后碰撞初期阶段.