大兴安岭北段新林地区晚石炭世花岗岩的岩石成因及地质意义

大兴安岭北段新林地区晚古生代花岗岩主要出露在大乌苏和富西里附近，岩性主要为二长花岗岩，另有少量花岗闪长岩。对其中二长花岗岩样品进行LA-ICP-MS锆石U-Pb测年表明，大乌苏和富西里岩体侵位年龄分别为（303.7±2.2）和（300.5±0.5）Ma，均为晚石炭世岩浆活动的产物。花岗岩具有富硅（w（SiO₂）为66.77%~75.85%）、富碱（w（Na₂O+K₂O）为7.41%~8.69%）、高铝（w（Al₂O₃）为12.90%~16.22%），低MgO、CaO、TiO₂的特点，属于钙碱性系列；铝饱和指数（A/CNK）为1.06~1.44，为过铝质岩石；镜下未见原生白云母、堇青石、石榴石等富铝矿物，不同于富铝的S型花岗岩；而w（P₂O₅）与w（SiO₂）负相关，呈I型花岗岩特征；富集LREE和Ba、Rb、K等大离子亲石元素，亏损Nb、Ta、Ti等高场强元素，与后造山I型花岗岩特征相似，应形成于拉张的构造环境。花岗岩的⁸⁷Sr/⁸⁶Sr为0.712 938、¹⁴³Nd/¹⁴⁴Nd为0.512 386，（⁸⁷Sr/⁸⁶Sr）_i值为0.704 4，ε_Nd（t）值为-1.09，T_DM2=1 172 Ma，源区物质主要为中-新元古代从亏损地幔增生的地壳物质。结合区域研究成果，大兴安岭新林地区晚石炭世岩浆侵位活动与额尔古纳-兴安地块和松嫩地块碰撞拼合后岩石圈伸展环境有关。     

西藏江玛地区闪长玢岩锆石U-Pb年龄、地球化学特征及其地质意义

目前对班公湖-怒江蛇绿混杂岩带的中酸性侵入岩的报道相对较少，对其成因和形成机制的研究有助于揭示班公湖-怒江特提斯洋构造带的岩浆作用过程和动力学背景.对西藏班公湖-怒江蛇绿混杂岩带江玛地区的闪长玢岩进行了年龄分析、岩石地球化学研究.闪长玢岩的LA-ICP-MS锆石U-Pb年龄为121±1 Ma，表明其形成于早白垩世晚期.岩石地球化学特征表明，闪长玢岩具高Al₂O₃含量（18.2%~19.3%），属高铝玄武岩，岩石富集轻稀土元素和大离子亲石元素Rb、K等，亏损高场强元素Nb、Ta、Ti，属典型的岛弧玄武岩.综合区域地质资料认为，江玛地区闪长玢岩可能形成于俯冲带前缘增生的岛弧环境，是早白垩世期班公湖-怒江特提斯洋壳岩石圈南向俯冲消减背景下，板片脱水形成的流体与地幔楔橄榄岩发生交代作用的产物.      

冀北大滩盆地钾玄岩系列的厘定、岩石成因及与铀成矿关系

大滩盆地位于华北克拉通北缘隆起带和沽源—红山子铀成矿带西南段,盆地内五里营铀矿点赋存在义县期(早白垩世晚期)二长斑岩中。二长斑岩全岩为高钾、富碱、低钛、贫铁,富集轻稀土元素和大离子亲石元素,无明显Eu负异常,具有碱性系列和钙碱性系列的特征,属典型的钾玄岩系列;[n(⁸⁷Sr)/n(⁸⁶Sr)]_i为0.707290～0.707399(平均值为0.707343),[n(¹⁴³Nd)/n(¹⁴⁴Nd)]_i为0.511849～0.511895(平均值为0.511876),ε_Nd(t)值变化范围是-12.38～-11.49,[n(²⁰⁶Pb)/n(²⁰⁴Pb)]_i为17.236～17.343(平均值17.296),[n(²⁰⁷Pb)/n(²⁰⁴Pb)]_i为15.407～15.428(平均值为15.416),[n(<...     

桂南钦防构造带西南段印支早期花岗岩的成因:年代学和地球化学约束

对桂南钦防构造带西南段印支早期花岗岩的地质调查发现,该区存在含变质矿物和不合变质矿物两类花岗岩。岩石学、地球化学和Sr-Nd同位素的分析结果表明:这些花岗岩均属于高钾钙碱性系列岩石,在岩石化学成分上明显富集大离子亲石元素（LILE,包括Rb、Th、U、K和Pb等）,而相对亏损高场强元素（HFSE,包括Nb、Ta、P和Ti等）,并具有较高的锶同位素初始比值((⁸⁷Sr/⁸⁶Sr)_i=0.715 250⁰.716 316)和较低的ε_Nd（t）值（-10.80^-9.92）,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。锆石SHRIMP U-Pb定年结果显示,含紫苏辉石花岗斑岩和黑云母二长花岗岩的锆石²⁰⁶Pb/²³⁸U加权平均年龄分别为（248.8±1.9）Ma（MSWD=1.5）和（246.0±3.4）Ma（MSWD=2.2）。结合本区存在有早古生代MORB型变质基性火山岩和二叠纪弧后扩张中心环境形成的E-MORB型玄武岩的资料,认为扬子板块和华夏板块结合带（称之为钦—杭结合带）西南段有古生代洋盆的存在。本区印支早期花岗岩很可能是在扬子板块和华夏板块之间的洋壳岩石圈向北俯冲的地球动力学背景下,幔源岩浆诱发俯冲板片中古老地壳物质重熔并受到地幔源区岩浆不同比例的交代作用的产物。     

特提斯喜马拉雅带江孜地区古近纪地层源区分析——对造山带早期地壳加厚的制约

特提斯喜马拉雅北亚带江孜地区上古新统-下始新统甲查拉组记录了喜马拉雅碰撞造山带的早期地壳加厚和沉积历史。本文我们报道了甲查拉组详细的碎屑锆石U-Pb年龄和全岩Sm-Nd同位素数据。甲查拉组由青灰色厚层的岩屑砂岩夹泥岩组成,不整合覆盖在宗卓组之上,碎屑锆石主要的峰值介于350～80 Ma, 900～470 Ma以及1 300～950 Ma,次要的峰值介于2 800～1 500 Ma。全岩⁸⁷Sr/⁸⁶Sr介于0.707 505～0.713 174,¹⁴³Nd/¹⁴⁴Nd介于0.512 206～0.512 355,ε_Nd(0)介于-5.52～-8.43。甲查拉组物源区以再循环的日喀则弧前盆地和上三叠统郎杰学群为主,少量物质来自雅鲁藏布江缝合带。上述研究表明,甲查拉组沉积在周缘前陆盆地的背景下,且特提斯喜马拉雅北亚带在始新世期间经历了明显的地壳加厚。     

西藏泽当地区雅鲁藏布江蛇绿岩中的斜长花岗岩特征及构造环境

位于西藏泽当地区的雅鲁藏布江蛇绿岩在我国研究程度较高,该蛇绿岩东西延伸约2 000 km,代表了印度和亚洲之间消失的新特提斯洋,是确定上述两大板块间缝合线存在的重要岩石学标志。文章对西藏泽当地区雅鲁藏布江蛇绿岩带中的斜长花岗岩特征及构造环境进行了研究。斜长花岗岩具有钙碱性特征;轻重稀土分异明显,LREE富集,HREE亏损,具有明显的正Eu异常,大离子亲石元素Rb、K、Ba、Sr富集,高场强元素Nb、Ti亏损;地球化学研究表明,斜长花岗岩应形成于俯冲造山的构造环境下,是俯冲洋壳部分熔融的产物,具有岛弧特点。     

湘东北栗山铅锌铜矿床二长花岗岩岩石成因及其氧逸度对成矿的指示意义

栗山铅锌铜矿床位于江南造山带多金属成矿带中段,是湘东北地区近几年发现的一大型矿床。本文对矿区内的黑云母二长花岗岩和二云母二长花岗岩进行了岩石地球化学、Sr-Nd同位素和黑云母矿物成分研究。岩石地球化学特征显示,两种二长花岗岩均属于高钾钙碱性系列。其中,黑云母二长花岗岩属于正常S型花岗岩,富集Rb、Th、K大离子亲石元素,亏损Ba、Nb、Ta、Sr、P、Ti高场强元素,有弱的Eu负异常,源区岩石为变质杂砂岩;二云母二长花岗岩属于分异的S型花岗岩,富集Rb、Th、U、K大离子亲石元素,亏损Ba、Nb、Sr、P、Ti高场强元素,Eu负异常大于前者,源区岩石为变泥质岩和少量变质杂砂岩。黑云母二长花岗岩的(⁸⁷Sr/⁸⁶Sr)_i值在0.7143～0.7159之间,ε_Nd(t)值介于-9.4～-8.5,两阶段模式年龄t_DM2变化范围为1631～1703 Ma;二云母二长花岗岩的(⁸⁷Sr/⁸⁶Sr)_i值在0.7156～0.7...     

原特提斯洋俯冲起始——来自北祁连早寒武世弧前岩浆作用的新证据

形成于弧前背景的岩浆岩可能记录了俯冲起始的重要证据。本文以北祁连走廊南山蛇绿混杂岩带新识别出的富铌辉长岩和斜长花岗岩为研究对象,在野外观察的基础上,结合岩石学、地球化学、同位素地球化学和年代学的综合研究,对原特提斯洋初始俯冲的时间进行约束,并探讨其形成过程及构造意义。锆石U-Pb定年结果显示富铌辉长岩样品锆石加权平均²⁰⁶Pb/²³⁸U年龄为512±4 Ma;两个斜长花岗岩样品的锆石加权平均²⁰⁶Pb/²³⁸U年龄分别为522±3 Ma和519±1 Ma。全岩地球化学结果显示富铌辉长岩的Nb含量为7.49×10^-6～10.80×10^-6,TiO₂含量为1.50%～2.08%,Nb/U值为11.9～13.4,(Nb/La)_N>0.5,明显高于岛弧玄武岩,其εNd(t)值介于+4.38～+5.78之间。斜长花岗岩样品K₂O含量(0.31%～1.66%)和K₂O/Na<...     

西藏乃东地区雅鲁藏布江缝合带中蛇绿混杂岩的变质作用及岩石学特征

雅鲁藏布江缝合带位于青藏高原南部,是印度板块向欧亚板块俯冲的产物,代表着新特提斯洋岩石圈的残片。文章对西藏乃东地区雅鲁藏布江缝合带中蛇绿混杂岩的变质作用及岩石学特征进行了研究。该带总体呈近东西向延伸,受变地质体主要为晚侏罗—早白垩世泽当蛇绿岩。通过野外地质调查、岩相学及岩石地球化学分析,结合岩石成因研究及构造环境判别,认为泽当蛇绿岩由地幔橄榄岩、辉长质杂岩、镁铁质杂岩、海相沉积物及伴生铬铁矿和斜长花岗岩等组成,属低绿片岩相—高绿片岩相区域变质岩。     

西藏拉萨地块阿翁错北二长花岗岩成因：锆石U-Pb年代学、岩石地球化学及Hf同位素制约

西藏拉萨地块阿翁错—盐湖岩浆弧的成因是解决班公湖—怒江特提斯洋俯冲极性和时限的关键。本文选取阿翁错北岩体中的二长花岗岩进行岩相学、锆石U-Pb年代学、岩石地球化学与Hf同位素研究。结果表明：阿翁错北二长花岗岩的锆石²⁰⁶Pb/²³⁸U加权平均年龄为(107.0±0.5) Ma,MSWD＝2.6,属早白垩世晚期。样品表现为高硅、富钾的高钾钙碱性岩石系列,A/CNK值介于1.006～1.019之间,属弱过铝质;微量元素富集大离子亲石元素Rb、K、U、Th及轻稀土元素（LREE）,亏损Nb、Ta、P、Ti等高场强元素,具中等至弱的负Eu异常（δEu=0.55~0.78）,属弱过铝质未分异的I型花岗岩。二长花岗岩样品锆石初始Hf同位素ε_Hf(t)值除1颗锆石达11.1外,其他17颗锆石介于3.7~6.3之间,平均值5.0,Hf同位素二阶段模式年龄变化于928~765 Ma之间。基于同位素以及岩石地球化学数据,阿翁错北岩体很可能是新生地壳熔融产生的长英质岩浆与镁铁质岩浆发生不均一混合作用形成,并有少量幔源物质的参与。结合拉萨地块中北部岩浆岩Hf同位素研究分析,阿翁错—盐湖岩浆弧形成于班公湖—怒江特提斯洋后退式俯冲的构造体制下,阿翁错北岩体的形成时代（107~104 Ma）代表了由断离板片俯冲末期向碰撞环境转化的时限。     

粤西北大桂山岩体LA-ICP-MS锆石U-Pb测年、岩石成因及构造环境分析

为了加深对粤西北地区加里东期岩浆岩演化的认识,对粤西北地区大桂山岩体进行了岩石学、岩相学、同位素年代学、岩石地球化学及Sm-Nd同位素地球化学等研究。研究显示:大桂山岩体位于广东佛冈复式岩体的北西缘,岩性为中粗粒含斑黑云母正长花岗岩;利用LA-ICP-MS锆石U-Pb法测得其年龄为(445.9±3.6) Ma,属晚奥陶世侵入岩;岩石地球化学具有高硅、富钾、高分异指数和低钙、低镁、贫铁的特征;稀土元素总量中等-偏低,轻重稀土元素分馏程度较低,具明显的负Eu异常;微量元素表现为大离子亲石元素Rb、Th、U、K、La、Ce、Nd等的富集,亏损P、Sr、Ba、Ti等高场强元素;(⁸⁷Sr/⁸⁶Sr)_i为0.025 3~0.100 2,ε_Nd(t)值较高,为-3.15~-5.26,T_DM2值较低。以上特征表明大桂山岩体为高分异或低熔融程度的铝质A型花岗岩,其源区为成熟度较低的地壳物质或者是一定比例壳幔物质的混合,因造山带垮塌、软流圈上涌引起地壳伸展,由于地幔上隆,幔源基性岩浆岩和钾玄质岩浆岩上侵或底侵而形成。     

Cumulate-like textures and chemical relationships in the Bressanone (Brixen) Granodiorite (Eastern Alps, Italy)— A new genetic approach

The shallow level pluton of Bressanone is a Late Hercynian multiple intrusion into the South Alpine basement of the Eastern Alps. Most of this complex is composed of anatectic granodiorites and granites intruded in separate stocks 282 ± 14 Ma ago; gabbros and leucogranites occur in smaller quantities. The chronological intrusion sequence is: layered gabbro, granodiorites and granites, two-mica cordierite leucogranite and fayalite leucogranites.

The granodiorites and granites may contain hornblende or garnet. The hornblende and garnet rocks differ both in chemistry and (⁸⁷Sr/⁸⁶Sr)_i ratio, and may be identified as “I-type” and “S-type”, respectively, according to the Chappell-White classification.

Textural and chemical patterns show that the granites may be linked to the granodiorites by cumulate-like processes. The granodiorite → granite transition, attributed to filter pressing, expresses an increase in the liquid/xenolith ratio in a magma where the liquid fraction was a minimum melt and the solid fraction was restitic material.     

Modeling of subduction components in the Genesis of the Meso-Cenozoic igneous rocks from the South Shetland Arc, Antarctica

Isotope data and trace elements concentrations are presented for volcanic and plutonic rocks from the Livingston, Greenwich, Robert, King George and Ardley islands (South Shetland arc, Antarctica). These islands were formed during subduction of the Phoenix Plate under the Antarctica Plate from Cretaceous to Tertiary. Isotopically (⁸⁷Sr/⁸⁶Sr)_o ratios vary from 0.7033 to 0.7046 and (¹⁴³Nd/¹⁴⁴Nd)_o ratios from 0.5127 to 0.5129. εNd values vary from +2.71 to +7.30 that indicate asthenospheric mantle source for the analysed samples. ²⁰⁸Pb/²⁰⁴Pb ratios vary from 38.12 to 38.70, ²⁰⁷Pb/²⁰⁴Pb ratios are between 15.49 and 15.68, and ²⁰⁶Pb/²⁰⁴Pb from 18.28 to 18.81. The South Shetland rocks are thought to be derived from a depleted MORB mantle source (DMM) modified by mixtures of two enriched mantle components such as slab-derived melts and/or fluids and small fractions of oceanic sediment (EM I and EM II). The isotopic compositions of the subduction component can be explained by mixing between at least 4 wt.% of sediment and 96 wt.% of melts and/or fluids derived from altered MORB.     

西藏墨竹工卡地区早侏罗世花岗岩地球化学、岩石成因及其地质意义

为了研究南冈底斯晚三叠世-早侏罗世时期岩浆岩的成因类型和构造背景，针对墨竹工卡地区的松多黑云母二长花岗岩体进行岩相学、年代学、全岩地球化学研究.LA-ICP-MS锆石U-Pb定年结果显示，松多黑云母二长花岗岩结晶年龄为190.2±2.9 Ma，形成于早侏罗世.在地球化学组成上，黑云母二长花岗岩具有低TiO₂（0.68%~0.75%），富SiO₂（65.22%~66.13%）、Al₂O₃（16.26%~16.73%）、Na₂O（4.05%~4.29%）、K₂O（3.96%~4.24%）的特点，显示钾玄岩系列和弱过铝质（A/CNK=1.04~1.11）的主量元素地球化学特征；在微量元素蛛网图上，具有富集Rb、Th、K、Zr、Hf元素和亏损Ba、Nb、Ta、Sr、Ti、P元素的特征；锆石饱和温度介于805~835℃，FeOT/MgO比值高，样品显示出具有部分A型花岗岩特征.结合前人研究表明，晚三叠世-早侏罗世时期南冈底斯岩浆岩构造背景与新特提斯洋北向俯冲有关，松多黑云母二长花岗岩形成于新特提斯洋板片北向俯冲引起的弧后伸展环境；其成因与软流圈上涌导致幔源岩浆底侵引起下地壳的部分熔融有关.      

Mixing of asthenospheric and lithospheric mantle-derived basalt magmas as shown by along-arc variation in Sr and Nd isotopic compositions of Early Miocene basalts from back-arc margin of the NE Japan arc

We report trace element and Sr–Nd isotopic compositions of Early Miocene (22–18 Ma) basaltic rocks distributed along the back-arc margin of the NE Japan arc over 500 km. These rocks are divided into higher TiO₂ (> 1.5 wt.%; referred to as HT) and lower TiO₂ (< 1.5 wt.%; LT) basalts. HT basalt has higher Na₂O + K₂O, HFSE and LREE, Zr/Y, and La/Yb compared to LT basalt. Both suite rocks show a wide range in Sr and Nd isotopic compositions (initial ⁸⁷Sr/⁸⁶Sr (SrI) = 0.70389 to 0.70631, initial ¹⁴³Nd/¹⁴⁴Nd(NdI) = 0.51248 to 0.51285). There is no any systematic variation amongst the studied Early Miocene basaltic rocks in terms of Sr–Nd isotope or Na₂O + K₂O and K₂O abundances, across three volcanic zones from the eastern through transitional to western volcanic zone, but we can identify gradual increases in SrI and decreases in NdI from north to south along the back-arc margin of the NE Japan arc. Based on high field strength element, REE, and Sr–Nd isotope data, Early Miocene basaltic rocks of the NE Japan back-arc margin represent mixing of the asthenospheric mantle-derived basalt magma with two types of basaltic magmas, HT and LT basaltic magmas, derived by different degrees of partial melting of the subcontinental lithospheric mantle composed of garnet-absent lherzolite, with a gradual decrease in the proportion of asthenospheric mantle-derived magma from north to south. These mantle events might have occurred in association with rifting of the Eurasian continental arc during the pre-opening stage of the Japan Sea.     

Origin and geodynamic significance of Tertiary postcollisional basaltic magmatism in Serbia (central Balkan Peninsula)

Tertiary basaltic magmatism in Serbia occurred through three episodes: (i) Paleocene/Eocene, when mostly east Serbian mafic alkaline rocks (ESPEMAR) formed, (ii) Oligocene/Miocene, dominated by high-K calc–alkaline basalts, shoshonites (HKCA–SHO) and ultrapotassic (UP) rocks, and (iii) Pliocene episode when rocks similar to (ii) originated. In this study, the geodynamics inferred from petrogenesis of the (i) and (ii) episodes are discussed.

The ESPEMAR (62–39 Ma) occur mainly as mantle xenolith-bearing basanites. Their geochemical features, such as the REE patterns, elevated HFSE contents and depleted Sr–Nd isotope signatures, indicate a relatively small degree of melting of an isotopically depleted mantle source. Their mantle-normalized trace element patterns are flat to concave and “bell-shaped”, characteristic of an OIB source free of subduction component. ⁸⁷Sr/⁸⁶Sr_i and ¹⁴³Nd/¹⁴⁴Nd_i isotope ratios (0.7030–0.7047 and 0.5127–0.5129, respectively) indicate a depleted source for the ESPEMAR similar to the European Asthenospheric Reservoir (EAR).

The HKCA–SHO rocks (30–21 Ma) occur as basalts, basaltic andesites and trachyandesites. They show enrichment in LILE and depletion in HFSE with all the distinctive features of calc–alkaline arc-type magmatism. This is coupled with somewhat enriched Sr–Nd isotope signature (⁸⁷Sr/⁸⁶Sr_i=0.7047–0.7064, ¹⁴³Nd/¹⁴⁴Nd_i=0.5124–0.5126). All these features are characteristic of subduction-related metasomatism and fluxing of the HKCA–SHO mantle source with fluids/melts released from subducted sedimentary material.

UP rocks (35–21 Ma) appear as (i) Si-rich lamproites and related rocks and (ii) olivine leucitites and related rocks. UP rocks have high-LILE/HFSE ratios with enrichment for some LILE around 1000× primitive mantle, troughs at Nb and Ti, and peaks of Pb in their mantle-normalized patterns. They also show highly fractionated REE patterns (La/Yb up to 27, La_N up to 400). The isotopic ratios approach crustal values (⁸⁷Sr/⁸⁶Sr_i=0.7059–0.7115 and ¹⁴³Nd/¹⁴⁴Nd_i=0.5122–0.5126), and that signature is typical for ultrapotassic rocks worldwide.

The Paleocene/Eocene episode and formation of the ESPEMAR is referred to as asthenospheric-derived magmatism. This magmatism originated through passive riftlike structures related to possible short relaxational phases during predominantly collisional and compressional conditions. The Oligocene/Miocene episode and formation of HKCA–SHO and UP rocks were dominated by lithospheric-controlled magmatism. Its origin is connected with the activity of a wide dextral wrench corridor generated along the axis of the Dinaride orogen which collapsed in response to thickened crust caused by earlier compressional processes.

To explain conditions of these two magmatic events, a three-stage geodynamic model has been proposed: (1) subduction–termination/collision stage (Paleocene/Eocene), (2) collision stage (Eocene) and (3) postcollision/collapse stage (Oligocene/early Miocene).     

Pre-Cenozoic intra-plate magmatism along the Central Andes (17–34°S): Composition of the mantle at an active margin

Sr–Nd–Pb isotope ratios of alkaline mafic intra-plate magmatism constrain the isotopic compositions of the lithospheric mantle along what is now the eastern foreland or back arc of the Cenozoic Central Andes (17–34°S). Most small-volume basanite volcanic rocks and alkaline intrusive rocks of Cretaceous (and rare Miocene) age were derived from a depleted lithospheric mantle source with rather uniform initial ¹⁴³Nd/¹⁴⁴Nd ( 0.5127–0.5128) and ⁸⁷Sr/⁸⁶Sr ( 0.7032–0.7040). The initial ²⁰⁶Pb/²⁰⁴Pb ratios are variable (18.5–19.7) at uniform ²⁰⁷Pb/²⁰⁴Pb ratios (15.60 ± 0.05). A variety of the Cretaceous depleted mantle source of the magmatic rocks shows elevated Sr isotope ratios up to 0.707 at constant high Nd isotope ratios. The variable Sr and Pb isotope ratios are probably due to radiogenic growth in a metasomatized lithospheric mantle, which represents the former sub-arc mantle beneath the early Palaeozoic active continental margin. Sr–Nd–Pb isotope signatures of a second mantle type reflected in the composition of Cretaceous (one late Palaeozoic age) intra-plate magmatic rocks (¹⁴³Nd/¹⁴⁴Nd  0.5123, ⁸⁷Sr/⁸⁶Sr  0.704, ²⁰⁶Pb/²⁰⁴Pb  17.5–18.5, and ²⁰⁷Pb/²⁰⁴Pb  15.45–15.50) are similar to the isotopic composition of old sub-continental lithospheric mantle of the Brazilian Shield.

Published Nd and Sr isotopic compositions of Mesozoic to Cenozoic arc-related magmatic rocks (18–40°S) represent the composition of the convective sub-arc mantle in the Central Andes and are similar to those of the Cretaceous (and rare Miocene) intra-plate magmatic rocks. The dominant convective and lithospheric mantle type beneath this old continental margin is depleted mantle, which is compositionally different from average MORB-type depleted mantle. The old sub-continental lithospheric mantle did not contribute to Mesozoic to Cenozoic arc magmatism.     

Early Cretaceous gabbroic rocks from the Taihang Mountains: Implications for a paleosubduction-related lithospheric mantle beneath the central North China Craton

SHRIMP zircon U–Pb ages and geochemical and Sr–Nd–Pb isotopic data are presented for the gabbroic intrusive from the southern Taihang Mountains to characterize the nature of the Mesozoic lithospheric mantle beneath the central North China Craton (NCC). The gabbroic rocks emplaced at 125 Ma and are composed of plagioclase (40–50%), amphibole (20–30%), clinopyroxene (10–15%), olivine (5–10%) and biotite (5–7%). Olivines have high MgO (Fo = 78–85) and NiO content. Clinopyroxenes are high in MgO and CaO with the dominant ones having the formula of En_42–46Wo_41–50Fs_8–13. Plagioclases are dominantly andesine–labradorite (An = 46–78%) and have normal zonation from bytownite in the core to andesine in the rim. Amphiboles are mainly magnesio and actinolitic hornblende, distinct from those in the Precambrian high-pressure granulites of the NCC. These gabbroic rocks are characterized by high MgO (9.0–11.04%) and SiO₂ (52.66–55.52%), and low Al₂O₃, FeOt and TiO₂, and could be classified as high-mg basaltic andesites. They are enriched in LILEs and LREEs, depleted in HFSEs and HREEs, and exhibit (⁸⁷Sr/⁸⁶Sr)_i = 0.70492–0.70539, ε_Nd(t) = − 12.47–15.07, (²⁰⁶Pb/²⁰⁴Pb)_i = 16.63–17.10, Δ8/4 = 70.1–107.2 and Δ7/4 = − 2.1 to − 9.4, i.e., an EMI-like isotopic signatures. Such geochemical features indicate that these early Cretaceous gabbroic rocks were originated from a refractory pyroxenitic veined-plus-peridotite source previously modified by an SiO₂-rich melt that may have been derived from Paleoproterozoic subducted crustal materials. Late Mesozoic lithospheric extension might have induced the melting of the metasomatised lithospheric mantle in response to the upwelling of the asthenosphere to generate these gabbroic rocks in the southern Taihang Mountains.     

Geochemical constraints on the origin of bimodal magmatism at the Okinawa Trough, an incipient back-arc basin

The magmatism at the axial zone of the middle Okinawa Trough, a young continental back-arc basin, comprises a bimodal basaltic–rhyolitic suite, accompanied by minor intermediate rocks. We report major and trace element and Sr–Nd isotopic data for the intermediate to silicic suites, to provide constraints on their petrogenesis. The rhyolites, recovered as lava and pumice, fall into three geochemical groups (type 1, 2, and 3 rhyolites). Type 1 rhyolites have ⁸⁷Sr/⁸⁶Sr (0.7040–0.7042) and ¹⁴³Nd/¹⁴⁴Nd (0.5128–0.5129) identical to those of associated basalts, and are characterized by highly fractionated REE patterns. Petrogenesis of type 1 rhyolites is explicable in terms of fractional crystallization of the associated basalt. In contrast, type 2 rhyolites and andesite have slightly higher ⁸⁷Sr/⁸⁶Sr (0.7044–0.7047) but similar ¹⁴³Nd/¹⁴⁴Nd (0.5128) compared to those of the basalts. The compositions of type 2 rhyolite and andesite can be explained by assimilation and fractional crystallization (AFC) processes of the basalt magma; quantitative analysis suggests assimilation/fractional crystallization (M_a/M_c) ratios of ≤0.05. Hybrid andesite generated by mixing of evolved basalt and type 1 rhyolite is also present. We emphasize that mechanical extension in this part of the Okinawa Trough involves gabbroic lower crust that resulted from fractionation of mantle-derived basaltic magmas. Type 3 rhyolite occurs only as pumice, which makes its derivation questionable. This rhyolite has major and trace element compositions and Sr–Nd isotopic ratios, which suggests that it may be derived from volcanic activity on the southern Ryukyu volcanic front, and arrived in the Okinawa Trough by drifting on the Kuroshio Current.