Petrology,geochemistry, and tectonic significance of Mesozoic shoshonitic volcanic rocks,Luzong volcanic basin,eastern China

We studied the geochemical characteristics of three types of Mesozoic igneous rocks from the Luzong volcanic basin: basaltic trachyandesite at Shuangmiao, pyroxene monzonite at Bajiatan, and quartz-syenite (A-type granite) at Huangmeijian. Based on analyses of whole-rock major elements, all investigated rocks are enriched in K, Na, Ti, Al, but depleted in Ca, representing a shoshonitic series. Trace element analyses show that these rocks are characterized by enrichments of large-ion lithophile elements and high field strength elements. Positive Nb and Ta anomalies in the chondrite-normalized spider diagram indicate that the shoshonitic volcanic rocks share similar features with Nb-enriched basalts, which are different from normal island-arc volcanical rocks (they are typically strongly depleted in Nb and Ta). Bulk-rock chemical compositions and Sr–Nd isotopes indicate that the three types of igneous rocks are geochemically comagmatic, suggesting that the melts were derived from an enriched mantle reservoir. We postulate an extensional tectonic setting for the formation of Luzong volcanic basin, possibly related to subduction of a palaeo-Pacific plate beneath the east Chinese continent during the Yanshanian period (Cretaceous). Therefore, the petrogenetic features of those volcanic rocks as well as A-type granites in the Luzong basin indicate that the regional large-scale Fe–Cu–Au mineralization was associated with oceanic slab melting, but not delamination or recycling of the ancient lower continental crust, as previously proposed.     

新疆西天山尼勒克二叠纪火山岩的地球化学特征及构造意义

尼勒克二叠纪火山岩位于伊犁盆地北缘阿吾拉勒山西段,该地区发育一套完整的陆相二叠纪地层。本文对该区二叠纪中下统哈米斯特组和乌郎组火山岩进行了系统的岩相学、岩石地球化学和Sr-Nd同位素研究。尼勒克火山岩岩石类型多样,有玄武岩、安山岩、粗面岩、流纹岩等,地球化学特征复杂。上火山岩哈米斯特组火山岩具有典型的钾玄岩特征,高K₂O(2.81%~3.91%)、Sr(>1000×10^-6)、稀土(ΣREE>200×10^-6)和(La/Yb)_N比值(9.7~11.7),低Nb/La比值(大多<0.2)。下火山岩乌郞组可分为二组,一组具最低SiO₂,最高MgO,较低Sr(<500×10^-6)、稀土(50×10^-6<ΣREE<80×10^-6)和(La/Yb)_N比值(1.6~2.2),相对较高的Nb/La比值(>0.35),类似富集型洋中脊(E-MORB)特征,暗示其岩浆是浅部源区高程度部分熔融的产物;另一组火山岩有着相对较大的变化,总体特征介于哈米斯特组与第一组火山岩两者之间。尼勒克火山岩有高度亏损的Sr-Nd同位素组成,类似MORB,表明它们源自长期亏损的地幔源区。然而,火山岩高度亏损的Nb-Ta、分异且富集大离子亲石元素的特征暗示,地幔源区受到近期俯冲带流体的交代作用,并受部分熔融程度及结晶分异作用的控制,是不同批次岩浆作用在不同演化阶段的产物。尼勒克大规模玄武质火山岩的形成可能和塔里木二叠纪地幔柱活动有关。     

西天山石炭纪火山岩岩石学及Sr-Nd同位素地球化学研究

西天山石炭纪火山岩连续从玄武岩、粗面质玄武安山岩、粗面安山岩、粗面岩一直演化到流纹岩。不同地区火山岩的同位素组成差别很大。出露在拉尔敦达坂一带的晚石炭世粗面安山岩-粗面岩具有较高的εNd（t）值（＋2、68～＋4．29）和较高的初始^87Sr／^86Sr值（0、7015～0．7051）。新源城南的早石炭世粗面安山岩．流纹岩具有相对较低的εNd（t）值（-0．22～＋0．87）和变化较大的初始^87Sr／^86Sr值（0．7045～0．7068）。早石炭世玄武岩的初始^87Sr／^86Sr值较低且变化范围较小（0．7045～0．7058）,εNd（t）值高且变化范围大（＋0．89～＋3．04）。本文的同位素地球化学资料以及前期的年代学研究表明,西天山晚泥盆-早石炭世岛弧自西向东逐渐消亡,取而代之的是晚石炭世碰撞后富钾岩浆的喷发。     

Syntectonic growth strata and implications for late Cenozoic tectonic uplift in the northern Tian Shan,China

Propagation of faults and folds in the foreland basins of Tian Shan is an important process accommodating Cenozoic crustal shortening and mountain building, but little is known about the accurate time of the Cenozoic tectonic deformation. Based on growth strata and age determination, we show that syntectonic growth strata began to develop in the middle part of Tian Shan since 6 Ma ago. Geometry analysis indicates that formation of the growth strata is associated with progressive fold-limb rotation. Formation of the growth strata is contemporaneous with the tectonic deformation in the thrusting and folding zones. Together with the remarkable increase of sedimentation rate as well as the accumulation of coarse molasse deposits, we conclude that the late Cenozoic crustal shortening and mountain building in the region initiated since about 6 Ma and lasted to the early Pleistocene, as a consequence of intracontinental deformation within the India–Eurasia convergent system.     

Elemental and Nd-Sr isotopic geochemistry of granitoids from the West Junggar foldbelt (NW China), with implications for Phanerozoic continental growth

We report elemental and Nd-Sr isotopic data for two coeval postcollisional magmatic suites (∼300 Ma), the Miaogou and Karamay suites, from West Junggar foldbelt (NW China), aiming to determine their source regions and implications for continental growth. The Miaogou rocks, monzonitic to granitic in composition, show low Mg# (<0.30) and depletion of HFSE, and are highly depleted in isotopic compositions, with I_Sr ranging from 0.7035 to 0.7045, ε_Nd(300 Ma) from + 8.4 to + 6.6 and young Nd model ages (0.37-0.70 Ga) that coincide with the ages of the ophiolites in the area. These features suggest that the Miaogou rocks were derived essentially by partial melting of a juvenile basaltic lower crust formed in the early to middle Palaeozoic. The Karamay suite contains a variety of rock types from gabbroic diorite to granite. These rocks show enrichment of LREE and LILE and depletion of HFSE, and have chemical composition (e.g., Mg# = 0.63-0.41) rather different from the Miaogou suite, though isotopically they are indistinguishable. This suggests that the parental magma of the Karamay suite originated from melting of a young lithospheric mantle that had previously been metasomatized during the Palaeozoic subduction. The West Junggar magmatic rocks represent production of juvenile continental crust in the Phanerozoic, triggered by upwelling of asthenophere in an extensional regime. The basement beneath the area is dominated by “trapped” Palaeozoic arc series and oceanic crust.     

南天山南缘早二叠世酸性火山岩的地球化学、同位素年代学及其构造意义

南天山南缘小提坎里克组酸性火山岩高碱（Na2O＋K2O=6．03％～7．70％）且富钾（K2O／Na2O=2．35,5．03）,铝饱和指数A／CNK=0．89～1．59（平均1．24）,里特曼指数（σ）为1．48,属于过铝质高钾钙碱性火山岩系。轻稀土强烈富集,亏损Ba、Cs、Nb、Ta、Sr等大离子亲石元素,Hf、Zr等高场强元素基本无亏损,具有明显的负铕异常（δEu=0．44～0．66）,总体具有碰撞晚期花岗岩类的地球化学特点。岩石的Nd／Th（1．78—3．50）、Th／U（4．10～9．79）、La/Ta（17．69～35．77）和Nb／Ta（10．48～11．84）比值显示了壳源特征。对火山岩中锆石进行的激光探针等离子体质谱（LA-ICP—MS）U—Pb微区测定获得了289．4&#177;5．5Ma的形成年龄。结合区域构造分析认为,早二叠世初南天山地区处于碰撞晚期冲断造山作用阶段,小提坎里克组火山岩是对南天山造山带南缘碰撞晚期冲断变形和前陆盆地发育的响应。     

Petrogenesis and tectonic implications of the middle Silurian volcanic rocks in northern West Junggar,NW China

Zircon U–Pb geochronological and geochemical analyses are reported for a suite of the middle Silurian volcanic rocks from northern West Junggar (NW China), southern Central Asian Orogenic Belt (CAOB), with the aim to investigate the sources, petrogenesis, and tectonic implications. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb analysis from an andesite yielded a concordant weighted mean ²⁰⁶Pb/²³⁸U age of 429 ± 3 Ma, indicating the presence of middle Silurian volcanic rocks in northern West Junggar. The andesite is tholeiite series and characterized by minor variations in compositions (SiO₂ = 55.68–59.17 wt.%, Al₂O₃ = 14.56–17.7 wt.%, TiO₂ = 0.55–1.23 wt.%, Na₂O + K₂O = 3.46–7.16 wt.%, and P₂O₅ = 0.15–0.37 wt.%), with wider MgO content (2.18–6.48 wt.%) and Mg# (57.4–77.9). All andesitic rocks are enriched in large-ion lithophile elements (LILEs; e.g. Rb, Ba, K, and Th) and light rare earth elements (LREEs), but strongly depleted in some high field strength elements (HFSEs; e.g. Nb, Ta and Ti), with slight negative Eu anomalies (Eu/Eu* = 0.8–1). These features suggest that the andesitic magmas were derived from 2–8% partial melting of a garnet lherzolite depleted mantle source with subducted sediments metasomatized by slab-derived fluids. Combining the current study with those data in existing literature, we conclude that the middle Silurian volcanic rocks formed in an intra-oceanic subduction setting during consumption of the Irtysh–Zaysan Ocean, and further confirm the eastern extension of the early Palaeozoic Boshchekul–Chingiz volcanic arc of East Kazakhstan in China.     

哀牢山构造岩浆带晚二叠世-早三叠世火山岩特征及其构造环境

本文对中国云南哀牢山构造岩浆带内的雅轩桥、帽盒山、绿春火山岩等开展了相关研究.雅轩桥附近的火山岩为晚二叠世,岩性主要为橄榄粗安岩-玄武岩(少量安山岩),在TAS图上既有碱性又有亚碱性.钾含量较低(＜1.19％),为低钾钙碱-中钾钙碱性,Peacock碱钙指数以钙碱性为主.与MORB相比,其痕量元素蛛网图亏损Nb、Ta,而富集Pb,从Zr-V曲线呈平坦型,并且整体比MORB亏损.稀土元素配分模式与MORB相近,但略显轻稀土元素富集和重稀土元素亏损.在构造环境判别图上均位于火山弧环境.他郎河边(雅轩桥地区)火山岩为英安岩,属亚碱性,中钾钙碱性,Peacock指数为钙性.痕量元素蛛网图、REE模式图以及大地构造环境判别图,均表明其属于弧的构造环境.由上推测雅轩桥火山岩在晚二叠世属于弧火山岩.帽盒山玄武岩的锆石SHRIMP U-Pb测年结果为249±1.6Ma,为早三叠世.岩性为亚碱性钠长玄武岩,低钾钙碱性系列,Peacock碱钙指数以钙性为主.痕量元素蛛网图和REE配分模式图与MORB相比,LREE略微富集.在构造环境判别图中位于从E-MORB向岛弧过渡的构造环境.绿春地区流纹岩的锆石SHRIMP U-Pb年龄为247.3±1.8Ma,为早三叠世,属亚碱性,钾玄岩系列,Peacock碱钙指数为碱钙性.痕量元素蛛网图、REE配分模式图及大地构造环境判别图显示其为成熟岛弧向陆陆碰撞的过渡环境.结合前人研究,推测哀牢山洋在晚泥盆世形成,可能在石炭世-早二叠世(?)处于洋的扩张期.晚二叠世时,在哀牢山洋的西侧出现了雅轩桥的初始孤火山岩,预示着至少在这一时期,哀牢山洋已经开始俯冲.到三叠世早期(249±1.6Ma),在哀牢山洋的东侧出现了具有弧和MORB的双重特性的帽盒山玄武岩,可能指示此时哀牢山洋盆已经变小,或已转化为孤间或弧后盆地,洋的演化进入了晚期阶段,并且在局部地段,如绿春地区,此时(247.3±1.8Ma)已经进入到成熟岛弧向陆陆碰撞的过渡阶段.因此支持哀牢山洋在晚三叠世闭合的结论,亦符合上三叠统一碗水组不整合在哀牢山蛇绿混杂岩之上的事实.     

北山南部早二叠世A型流纹岩地球化学特征及其地球动力学意义

北山南部晚古生代构造背景争议已久,该区下二叠统普遍发育一套流纹岩系,但尚未对其开展系统的年代学与地球化学研究。本文首次对该区下二叠统5条代表性剖面中的流纹岩进行了元素地球化学、锆石U-Pb定年、全岩Sr-Nd及锆石Hf同位素等研究。U-Pb年龄介于294. 7～272. 7Ma之间,结合古生物资料,明确其喷发时代为早二叠世。该区早二叠世流纹岩大多属高钾钙碱性系列,呈钙碱-碱钙质,为准铝-过铝质岩石。岩石主量元素Si、K、Na、Fe含量高,贫Ca、Mg;微量元素富集Ga、Rb、Zr、Hf、Y,亏损Ba、Sr等;稀土元素存在明显负Eu异常;呈现高Fe OT/MgO与10000×Ga/Al比值,这些特征表明该区早二叠世流纹岩为A型流纹岩。独山剖面流纹岩锆石具有高的εHf(t)值(+11. 4～+16. 7),Hf模式年龄(tDM2)为231～573Ma;干泉剖面流纹岩锆石εHf(t)值为-8. 8～+8. 9,Hf模式年龄(tDM2)为802Ma～1961Ma;珊瑚井和俞井子剖面中的流纹岩具有低的εNd(t)值(-4. 21～-2. 92),高的(87Sr/86Sr)i值(0. 7108～0. 7223),Nd亏损地幔模式年龄(tDM)为1256～1386Ma,其锆石εHf(t)为正值(+1. 3～+17. 0)(tDM2为691～1797Ma);沙红山南剖面流纹岩εNd(t)值(-2. 00～-1. 81)较低,Nd亏损地幔模式年龄(tDM)为1207～1222Ma,锆石εHf(t)值介于+1. 6～+10. 7(tDM2为623～2576Ma)。地球化学特征表明独山剖面的流纹质岩浆为古生代新生地壳的重熔,其它剖面酸性火山岩岩浆则主要为中新元古代地壳物质熔融所形成,揭示了北山南部早二叠世岩石圈伸展减薄的地球动力学背景,结合该区构造岩浆事件序列、地层学以及沉积学等其它地质学证据,认为北山南部早二叠世为裂谷环境。     

Geochronology and geochemistry of late Carboniferous volcanic rocks from northern Inner Mongolia,North China: Petrogenesis and tectonic implications

Zircon U–Pb ages, geochemical and Sr–Nd isotopic data are presented for the late Carboniferous Baoligaomiao Formation (BG Fm.) and Delewula Formation (DW Fm.) volcanic rocks, widely distributed in northern Inner Mongolia, in the northern part of the Xing'an–Mongolia Orogenic Belt (XMOB). The BG Fm. rocks mainly consist of basaltic andesites and andesites while the DW Fm. rocks include dacites, trachytes, rhyolites, pyroclastic rocks and minor andesites. New LA-ICPMS zircon U–Pb analyses constrain their eruption to late Carboniferous (317–322 Ma and 300–310 Ma, respectively). The BG Fm. volcanic rocks are characterized by enriched large ion lithophile elements (LILE) and depleted high field strength elements (HFSE), with initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70854–0.70869 and negative εNd(t) (− 2.1 to − 2.4) values. They have low La/Ba (0.03–0.05), high La/Nb (2.05–3.70) ratios and variable Ba/Th (59.5–211) ratios. Such features suggest that they are derived from melting of heterogeneous sources including a metasomatized mantle wedge and Precambrian crustal material. The DW Fm. volcanic rocks are more depleted in HFSE with significant Nb, Ta, P, Ti anomalies. They have high initial ⁸⁷Sr/⁸⁶Sr ratios (0.72037–0.72234) and strong negative εNd(t) (− 11 to − 11.6) values which indicate those igneous rocks were mainly derived from reworking of the Paleoproterozoic crust. The late Carboniferous volcanic rocks have geochemical characteristics similar to those of the continental arc rocks which indicate the northward subduction of the Paleo Asian Ocean may have continued to the late Carboniferous. The volcanic association of this study together with the early Permian post-collisional magmatic rocks suggests that a tectonic transition from subduction-related continental margin arc volcanism to post-collisional magmatism occurred in the northern XMOB between the late Carboniferous and the early Permian.     

Petrology,geochemistry and tectonic significance of serpentinized ultramafic rocks from the South Arm of Sulawesi,Indonesia

Serpentinized ultramafic rocks occur in two separate basement complexes in the South Arm of Sulawesi, the Bantimala and Barru Blocks. We present petrographic, mineral chemical and geochemical data for these rocks, and interpret them in terms of petrogenesis and tectonic setting. The rocks of both blocks show strong serpentinization of original anhydrous silicates. The Bantimala ultramafics consist mainly of peridotite (harzburgite and dunite) and clinopyroxenite, with lenses of podiform chromitite. Metamorphism is evidenced by the occurrence of amphibolite-facies tremolite schist. In contrast, the Barru ultramafics consist of harzburgite peridotite and podiform chromitite, which also show an amphibolite-facies overprint that in this case may be related to intrusion by a large dacite/granodiorite body. Whole-rock trace element analyses and spinel compositions show that the Barru harzburgite is depleted relative to primitive mantle, and has had some melt extracted. In contrast, the Bantimala dunite, harzburgite and clinopyroxenite are cumulates. Both are derived from a supra-subduction zone environment, and were obducted during the closure of small back-arc basins. If there has been no rotation of the blocks, then the Bantimala ultramafics were emplaced from an ENE direction, while the Barru ultramafics were emplaced from the WNW. The ultramafic suites from these two blocks are juxtaposed with metamorphic assemblages, which were later intruded by younger volcanics, particularly in the Barru Block.     

Permian bimodal dyke of Tarim Basin, NW China: Geochemical characteristics and tectonic implications

This study reports for the first time the occurrence of bimodal dyke in the Shuigongtuan area of Bachu County, Tarim Basin, NW China. Here, quartz syenite porphyry and diabase dykes occur in direct contact showing bimodal feature. The quartz syenitic porphyry is metaluminous, enriched in K₂O + Na₂O (10–11 wt.%) and total rare earth elements (REE), with low Mg/(Mg + Fe) ratios, high LREE/HREE, and negative Eu anomalies. The chemical characteristics and tectonic discriminative diagrams show that the rocks have geochemical affinity with A-type granites. The diabase dyke shows 45–52 wt.% SiO₂ and Mg/(Mg + Fe) ratio in the range of, with high total REE, high LREE/HREE ratios and lack of Eu anomalies, broadly corresponding to tholeiitic composition. Based on low Y/Nb (as low as 0.4, and less than 1.2), enrichment in LILE and HFSE, and uniform Nb-enrichment patterns in spider diagram for the quartz syenitic porphyry, together with the geochemical patterns of the diabases, this biomodal association is interpreted to be derived from a mantle source and formed under typical within-plate environment. The quartz syenitic porphyry and diabase have Daly gap of 46 wt.%–67 wt.% in SiO₂, which is explained through formation under bimodal rifting. The quartz syenitic dyke probably formed during Early Permian (277 Ma) and has geochemical affinity with the Xiaohaizi syenitic body. We propose that magmas sourced from the mantle intruded into middle–upper crust and were emplaced as dykes, which indicate large-scale extension during the Permian in Tarim Basin. The bimodal dyke has genetic affinity with the huge volume of Permian basalts and igneous rocks (248–292 Ma) that occur in the Tarim Basin. The magmatism manifests the culmination of the major thermal event in the Tarim Basin.     

南秦岭富钠长石岩石的岩石学和地球化学研究

钠长石岩是秦岭造山带中的一类特色岩石,主要分布于南秦岭凤-太、山-柞和镇-旬盆地的泥盆纪和志留纪地层中,与秦岭众多大中型铅锌矿、金矿成矿关系密切。钠长石岩呈层状、似层状产于碎屑岩或碳酸盐岩岩系中,与区域地层整合产出,渐变过渡。具有条带状、块状、角砾状构造;XRF分析数据显示钠长石岩化学成分以富Na2O、Al2O3、SiO2等常量元素及Pb、Zn、Au、Ag、Cu等微量元素为特征;富含元素Ba和Fe-Mn-(Co+Cu+Ni)×10分布图投影表明钠长石岩具有热水沉积的特征。而TiO2与Al2O3关系图解又反映出钠长石岩中有陆源碎屑物混入的特点,其Ti、Al可能来自水成沉积物中的粘土;ICP-MS分析所得稀土元素数据显示,钠长石岩与地层稀土元素在稀土总量、轻重稀土量和配分模式均十分相似,其较高∑REE就是成岩过程中水成沉积物混入造成的。富钠长石岩石在形成时间上发生在沉积和早期成岩阶段,与正常沉积岩同时形成。兼具有正常沉积岩石和热水沉积岩石的特点,是一种有别于二者的混合型非典型热水沉积岩石。研究认为钠长石岩是由一种富钠和金属元素的热水通过对地层元素的萃取并与海底未固结沉积物混合而成的混合热液经沉积成岩作用形成的。     

华北板块北缘东段二叠纪的构造属性:来自火山岩锆石U-Pb年代学与地球化学的制约

本文对华北板块北缘东段大河深组、关门咀子组火山岩进行了锆石LA-ICP-MSU-Pb定年和岩石地球化学研究以便制约该区的区域构造演化。大河深组和关门咀子组火山岩中的锆石均呈自形-半自形晶,具有清晰的岩浆振荡生长环带和条痕状吸收(玄武安山岩)的特点,其Th/U比值高达0.31～1.56,表明其岩浆成因。测年结果表明,桦甸大河深组流纹岩形成于早二叠世(279±3Ma～293±2Ma),珲春和图们关门咀子组玄武安山岩和玄武岩分别形成于早二叠世(275±7Ma)和晚二叠世(250±5Ma)。大河深组火山岩SiO2含量介于64.9%～75.4%,Mg#值介于0.21～0.57,属于中钾-高钾钙碱性系列,明显富集轻稀土元素(LREEs)和大离子亲石元素(LILEs)、亏损高场强元素(HFSEs,如Nb、Ta、Ti)以及P元素,类似活动大陆边缘火山岩;其锆石的εHf(t)值为+0.9～+10.37,Hf同位素二阶段模式年龄值为785～1240Ma,表明它们均起源于中-新元古代新增生玄武质下地壳的部分熔融。珲春早二叠世关门咀子组属于中钾钙碱性系列;贫硅(53.4%～53.7%)和HFSEs,富铝(16.4%～16.8%)和LILEs,具有较低的稀土元素总量,以及较平坦的稀土配分型式,显示出岛弧火山岩的地球化学属性;该组火山岩的原始岩浆应起源于受俯冲板片脱水熔融交代的亏损地幔楔。图们晚二叠世关门咀子组玄武岩SiO2含量为48.7%～49.6%,Mg#值高达0.64～0.72,相对富集LREEs和LILEs,亏损HREEs和HFSEs,具有火山弧玄武岩的地球化学属性,同时其εNd(t)=+6.01,暗示其原始岩浆起源于亏损的岩石圈地幔。综上所述,我们认为早二叠世至晚二叠世期间,华北板块北缘东段(吉林中部地区)和兴凯地块西南缘均处于古亚洲洋的俯冲作用下。     

松辽盆地早白垩世火山岩地球化学特征及其构造意义

松辽盆地早白垩世发育基性岩类的橄榄玄武岩、玄武岩,中性岩类的玄武安山岩、安山岩、粗安岩,酸性岩类的粗面岩、粗面英安岩、英安岩和流纹岩。早白垩世火山岩都具有大离子亲石元素(LILE)、轻稀土元素(LREE)相对富集的特征,并具有较低的(~(87)Sr/~(86)Sr)_i值和较高的(~(143)Nd/~(144)Nd)_i值。基性岩类Ce/Nb=1.92～8.31,Th/Nb=0.08～0.44,(~(87)Sr/~(86)Sr)_i=0.7031～0.7047,ε_(Nd)(t)=+1.7～+5.2;中性岩类Ce/Nb=3.70～15.80,Th/Nb=0.34～2.58,(~(87)Sr/~(86)Sr)_i=0.7040～0.7054,ε_(Nd)(t)=0～+3.0;酸性岩类Ce/Nb=4.29～15.80,Th/Nb=0.11～1.02,(~(87)Sr/~(86)Sr)_i=0.7038～0.7066,ε_(Nd)(t)=+1.0～+3.3。基性岩类岩浆主要来自被富集了的亏损地幔源的部分熔融,受混染程度很小;中性岩类岩浆源于年轻地壳组分被来自亏损地幔岩浆的再次部分熔融;酸性岩类浆岩可能主要来自源于亏损地幔的初生地壳部分熔融,并受到了上地壳的混染。晚侏罗世-早白垩世,由于蒙古-鄂霍次克洋关闭引发强烈的挤压造山后,发生的岩石圈拆沉作用致使来自亏损地幔的岩浆熔融上涌导致岩石圈主动伸展拉张,地表则表现出大规模的裂谷断陷作用和大范围的火山活动。这个阶段的伸展拉张以主动拉张为主。     

朝鲜半岛咸北地区晚二叠世基性火山岩的地球化学特征:岩石成因与地质意义

晚古生代火山作用既是探究朝鲜半岛北部大地构造归属的基本要素,又是认识东北亚大陆两大构造域转折之际地球动力学过程的关键窗口。针对咸北地区会宁基性火山岩开展的离子探针锆石U-Pb年代学和地球化学研究表明,其喷发时代为晚二叠世(257±3Ma);主要岩石类型包括玄武安山岩和安山岩,其Si O2含量介于51.9%~55.3%,Mg#变化于40~58,属于钙碱性系列;岩石富集大离子亲石元素、亏损高场强元素(如Nb,Ta,Ti),并具有高正的锆石εH f(t)(+10~+16)值。元素-同位素地球化学示踪指示该基性火山岩的岩浆源区为俯冲沉积物熔体交代而形成的不均一岩石圈地幔,其部分熔融则可能归因于俯冲后的板内伸展背景。会宁基性火山岩不仅为判定咸北地块属于中亚造山带在朝鲜半岛的东延部分提供了岩石学证据,而且与中国相邻地块上的同期岩浆事件一道,构成见证古亚洲洋东段沿西拉木伦-长春-延边一线断裂带最终闭合的空间标记。     

The geochemistry of the potassium-rich Permian volcanic rocks of Devonshire,England

This paper re-defines the Permian volcanics of Devonshire as belonging to the high-potassium shoshonitic suite of rocks commonly associated with a late or post-orogenic tectonic setting. Factor analysis is used to classify the rocks into four well defined groups based upon major and extensive trace element data. They are interpreted on an Andean-type plate collision model as being derived from a subduction zone at a depth of some 300 km beneath Hercynian Europe.     

Petrology and geochemistry of boninite series volcanic rocks from the Mariana trench

Boninite series volcanic rocks have been recovered from three dredge hauls on the inner slope of the Mariana Trench. These hauls included olivine boninites, boninites, boninitic andesites and boninitic dacites, as well as island arc tholeiitic basalts and andesites. The boninite series volcanics range from 52 to 68% SiO₂, and are characterized by very low abundances of high-field-strength cations and heavy-rare-earth elements. Boninites and olivine boninites have phenocrysts of olivine and orthopyroxene, the andesites phenocrysts of orthopyroxene and clinopyroxene, and the dacites orthopyroxene, clinopyroxene, and plagioclase. Most of the major and trace element variation in the series from boninite to boninitic dacite can be modelled by fractionation of olivine, orthopyroxene, clinopyroxene, and plagioclase in the proportions 2.5412, leaving 47% residual liquid. The fractionation must be in part open-system: reverse zoned phenocrysts, resorbed olivine and plagioclase xenocrysts, and bulk rock compositions which cannot be fit by simple closed system crystallization indicate some magma mixing and phenocryst accumulation. Two boninitic magma stems can be identified, with similar high-field-strength element abundances, but different amounts of Ca, Na, Al and light-rare-earth elements. There is also evidence for a magma stem transitional in chemistry from the boninites to arc tholeiites. The compositions of these boninites are consistent with hypotheses for boninite formation by partial melting of a depleted mantle mixed with an incompatible element enriched fluid. The Mariana forearc boninite series lacks a strong iron enrichment, but produces andesites with lower Ti, Al and Y/Zr, and higher Mg, Ni and Cr than typical calcalkaline arc andesites and dacites. Boninites in the Mariana system were erupted only in the earliest phases of subduction zone activity.     

Geochemistry and geochronology of Carboniferous volcanic rocks in the eastern Junggar terrane,NW China: Implication for a tectonic transition

The Carboniferous tectonic setting of the Junggar terrane, northern Xinjiang, NW China, has long been a matter of debate. Voluminous Carboniferous volcanic rocks are widely distributed in the Karamaili area, the southern part of the eastern Junggar terrane. Early Carboniferous rocks comprise basalts and basaltic andesites, with enrichment of LREE and LILE and depletion of HFSE, and uniformly high ε_Nd(t) (+ 3.7 to + 4.0). Late Carboniferous rocks consist of basalts, basaltic andesites, rhyolites and minor dacites, and can be subdivided into basic and felsic groups. The basic rocks are depleted in HFSE, and show variable high ε_Nd(t) (+ 4.8 to + 6.9). They have higher Cr and Ni and lower Na₂O, U and Th contents than early Carboniferous basic rocks. The felsic rocks show A-type affinity, with typical enrichment of alkalis, LREE and HFSE and strong depletion in Ba, Sr, Eu and Ti. They have high values of ε_Nd(t) and zircon ε_Hf(t) (+ 11.6 to + 17.9). New LA-ICPMS zircon U–Pb analyses constrain their emplacement to late Carboniferous time (306.5–314.3 Ma).The Carboniferous basic rocks show negative Zr-Hf anomalies and low Th/Ce (< 0.07) and Th/La (0.06–0.16), excluding significant crustal contamination during magma evolution. They have low La/Ba (0.03–0.12), Ce/Y (< 3) and (Tb/Yb)_N (< 2) and variable Ba/Th (28–318) and Ba/La (3.1–34), suggesting that they were derived from a main spinel with minor garnet lherzolite mantle source metasomatized by slab-derived fluids. The late Carboniferous felsic rocks were produced when upwelling asthenosphere triggered partial melting of juvenile lower crust. The early Carboniferous volcanism occurred in an island-arc setting related to the southward subduction of the Paleo-Junggar Ocean plate, whereas the late Carboniferous rocks erupted in a post-collisional extensional setting. Thus, a rapid tectonic transition from arc to post-collisional extension may have occurred between early and late Carboniferous, and probably resulted from slab break-off or lithospheric delamination.