The Mayari-Baracoa Paired Ophiolite Belt,Eastern Cuba: Implications for Tectonic Settings and Platinum-Group Elemental Mineralization

The Jurassic Mayari-Baracoa ophiolite belt and associated Cretaceous volcanic rocks form the Zaza zone of eastern Cuba. This zone has been traditionally considered allochthonous and overrides a passive continental margin, the Cuban foreland. The ophiolites consist of mantle tectonites and cumulates, overlain by a volcanicarc sequence including porphyritic basalts and andesitic lavas. These are, in turn, overlain by a sequence of tuffs and epiclastic sedimentary rocks. There are two ophiolitic massifs in the belt, the Mayari-Cristal Massif (MCM) and the Moa-Baracoa Massif (MBM). The MCM consists of harzburgites and dunites with abundant high-Cr podiform chromitites and dikes of gabbro and pyroxenite. The MBM, on the other hand, is composed of harzburgites with abundant high-Al podiform chromitites, cut by troctolite dikes. The two ophiolitic massifs have different REE and PGE patterns and contents. The mantle sequence in the MCM is more depleted than that in the MBM. We suggest that the MCM formed beneath a volcanic island arc and the MBM beneath a nascent spreading center in a back-arc basin. The two massifs form a paired ophiolite belt.     

陕西商州地区丹凤变质火山岩的地球化学特征

陕西商州地区丹凤变质火山岩具有洋内岛弧火山岩地球化学特征,它们是分别来自不同源区的拉斑玄武和钙碱性2个系列共存的一套变质火山岩。其Th/Ta比值高及Ni、Ta、Ti、Y和Yb含量低,表明岩石受到消减带组分的影响。种种证据表明,丹凤变质火山岩是早古生代华北地块南缘消减带之上洋内岛弧环境的产物。     

Neoproterozoic nascent island arc volcanism from the Nubian Shield of Egypt: Magma genesis and generation of continental crust in intra-oceanic arcs

The Neoproterozoic Wadi Ranga metavolcanic rocks, South Eastern Desert of Egypt, constitute a slightly metamorphosed bimodal sequence of low-K submarine tholeiitic mafic and felsic volcanic rocks. The mafic volcanic rocks are represented by massive and pillow flows and agglomerates, composed of porphyritic and aphyric basalts and basaltic andesites that are mostly amygdaloidal. The felsic volcanic rocks embrace porphyritic dacites and rhyolites and tuffs, which overlie the mafic volcanic rocks. The geochemical characteristics of Wadi Ranga volcanic rocks, especially a strong Nb depletion, indicate that they were formed from subduction-related melts. The clinopyroxene phenocrysts of basalts are more akin to those crystallizing from island-arc tholeiitic magmas. The tholeiitic nature of the Wadi Ranga volcanics as well as their LREE-depleted or nearly flat REE patterns and their low K₂O contents suggest that they were developed in an immature island arc setting. The subchondritic Nb/Ta ratios (with the lowest ratio reported for any arc rocks) and low Nb/Yb ratios indicate that the mantle source of the Wadi Ranga mafic volcanic rocks was more depleted than N-MORB-source mantle. Subduction signature was dominated by aqueous fluids derived from slab dehydration, whereas the role of subducted sediments in mantle-wedge metasomatization was subordinate, implying that the subduction system was sediment-starved and far from continental clastic input. The amount of slab-derived fluids was enough to produce hydrous magmas that follow the tholeiitic but not the calc-alkaline differentiation trend. With Mg# > 64, few samples of Wadi Ranga mafic volcanic rocks are similar to primitive arc magmas, whereas the other samples have clearly experienced considerable fractional crystallization.The low abundances of trace elements, together with low K₂O contents of the felsic metavolcanic rocks indicate that they were erupted in a primitive island arc setting. The felsic volcanic rocks are characterized by lower K/Rb ratios compared to the mafic volcanic rocks, higher trace element abundances (~ 2 to ~ 9 times basalt) on primitive arc basalt-normalized pattern and nearly flat chondrite-normalized REE patterns, which display a negative Eu anomaly. These features are largely consistent with fractional crystallization model for the origin of the felsic volcanic rocks. Moreover, SiO₂-REE variations for the Wadi Ranga volcanic rocks display steadily increasing LREE over the entire mafic to felsic range and enriched La abundances in the felsic lavas relative to the most mafic lavas, features which are consistent with production of the felsic volcanic rocks through fractional crystallization of basaltic melts. The relatively large volume of Wadi Ranga silicic volcanic rocks implies that significant volume of silicic magmas can be generated in immature island arcs by fractional crystallization and indicates the significant role of intra-oceanic arcs in the production of Neoproterozoic continental crust. We emphasize that the geochemical characteristics of these rocks such as their low LILE and nearly flat REE patterns can successfully discriminate them from other Egyptian Neoproterozoic felsic volcanic rocks, which have higher LILE, Zr and Nb and fractionated REE patterns.     

内蒙朝克山蛇绿岩地球化学: 洋内弧后盆地的产物？

朝克山蛇绿岩是内蒙贺根山地区出露最好的蛇绿岩之一,可能形成于中晚石炭世。朝克山蛇绿岩中的基性岩具有LREE亏损、类似N-MORB的稀土配分模式,而相对N-MORB富集大离子亲石元素,亏损Nb、Ta等高场强元素又类似岛弧火山岩的成分特征,因此,我们认为朝克山蛇绿岩应形成于弧后盆地。将朝克山蛇绿岩的基性岩与现代Mariana洋内弧后盆地和Okinawa陆缘弧后盆地的玄武岩及同属中亚造山带的、形成于洋内弧后盆地的新疆库尔提蛇绿岩对比,朝克山蛇绿岩更类似于Mariana玄武岩和库尔提蛇绿岩,因此其很可能形成于洋内弧后盆地而不是大陆边缘弧后盆地环境。     

Paleoproterozoic arc and ophiolitic rocks on the northwest-margin of the Trans-Hudson Orogen,Saskatchewan, Canada: their contribution to a revised tectonic framework for the orogen

A new lithotectonic framework for the northwestern Reindeer Zone of the Trans-Hudson Orogen divides rocks into five northwest- to north-dipping volcano-sedimentary assemblages: (1) at the structural base, the 1.92–1.87 Ga largely sedimentary Levesque Bay Assemblage (partly equivalent to former ‘MacLean Lake gneisses’), which lies within the confines of the Kisseynew Domain and is tectonically imbricated with metasedimentary rocks of the <1.85 Ga McLennan and Burntwood groups; (2) the turbiditic Duck Lake Assemblage, also located along the northern edge of the Kisseynew Domain; it contains detrital zircons ranging in age between 1.92 and 1.87 Ga; (3) the ?1.92 Ga mafic–ultramafic volcano-plutonic Lawrence Point Assemblage of the La Ronge Domain; (4) the ≥1.88 Ga felsic to intermediate volcano-plutonic Reed Lake Assemblage of the La Ronge Domain; and (5) the turbiditic Milton Island Assemblage of the Rottenstone Domain, which contains detrital zircons ranging in age between 2.83 and 1.86 Ga. The assemblages are intruded by a variety of 1.91–1.78 Ga mafic to felsic plutons.The Lawrence Point Assemblage is interpreted as a dismembered supra-subduction zone ophiolite. High-MgO refractory harzburgite (‘Group 1’ ultramafic rocks), at the structural base of the assemblage, is geochemically identical to the upper mantle section of selected supra-subduction zone ophiolites and mantle tectonites. Chromite and olivine compositions of the ‘Group 1’ ultramafic rocks are also comparable to those of ophiolitic harzburgite and mantle tectonite. Mafic metavolcanic rocks of the assemblage are classified as subalkaline tholeiitic basalts. Their trace element patterns and Hf, Ta, Th, Y, Nb, and La element ratios resemble those of modern back-arc basin basalts. The Reed Lake Assemblage represents a subduction-generated arc complex that was built on top of the Lawrence Point Assemblage; its mafic metavolcanic rocks are subalkaline basalts, with calc-alkaline trends, and elevated Th and Ce concentrations and negative Nb anomalies. Feldspar porphyry dykes intruding the Lawrence Point and Duck Lake assemblages constrain timing of Lawrence Point ophiolite emplacement onto the Duck Lake Assemblage to 1.86–1.84 Ga. The trace element geochemistry of the dykes suggests continued arc volcanism after ophiolite emplacement. Mafic metavolcanic rocks of the Levesque Bay Assemblage are geochemically similar to those of the Lawrence Point Assemblage. Other ultramafic rocks (peridotite to pyroxenite) are abundant in the Lawrence Point Assemblage, but have similar geochemistry to small ultramafic bodies intruding the Reed Lake, Duck Lake and Levesque Bay Assemblages. They represent a separate, later phase (?1.86 Ga) of ultramafic plutonism, which post-dates ophiolite emplacement.Timing of Lawrence Point ophiolite emplacement (between 1.86 and 1.84 Ga) and geochemistry of later felsic and mafic/ultramafic volcanism suggest that the Lawrence Point ophiolite and overlying Reed Lake arc assemblage were not accreted to the Hearne Craton prior to 1.86 Ga, but were first accreted to the Flin Flon–Glennie Complex after 1.86 Ga.     

西藏雅鲁藏布江缝合带西段南北亚带蛇绿岩的成因探讨

雅鲁藏布江蛇绿岩带自萨嘎以西分为达巴-休古嘎布(南亚带)和萨嘎-达机翁(北亚带)两个亚带,但两者的关系和构造环境还存在争论。本文在实测剖面的基础上,对比南北亚带蛇绿岩产出和岩石组成,以及对比基性岩脉和围岩方辉橄榄岩的地球化学和年代学特征,探讨两个带的关系和构造环境。北带错不扎、加纳崩和巴尔基性岩脉具有高Si、Al,低K、Ti、P特征,属钙碱性玄武质成分,锆石U-Pb年龄为125~128Ma;南亚带东波、普兰、休古嘎布基性岩具有高Mg、Ti,低Si、K特征,为低钾拉斑玄武质成分,锆石U-Pb年龄为120~130Ma。南北带基性岩具有类似产状和年代学,球粒陨石标准化曲线与NMORB一致;N-MORB标准化蛛网图中显示Nb、Ta、Ti负异常,指示洋内俯冲带弧前或弧后环境。此外,两带基性岩围岩方辉橄榄岩中橄榄石、辉石、尖晶石具有类似的矿物成分特征,地球化学成分兼有弧前和深海地幔橄榄岩的特点,指示南北亚带蛇绿岩形成于俯冲带弧前环境。结合区域对比,南亚带蛇绿岩不发育沟-弧-盆体系,具有从北到南的构造侵位特征,南北亚带之间的仲巴微地体具有特提斯喜马拉雅的亲缘性,指示南北蛇绿岩亚带可能为相同大洋岩石圈的不同残余。     

Evolution of an intra‐oceanic island arc during the Late Silurian to Late Devonian,New England Fold Belt

Geochemical studies of volcanic rocks in the Gamilaroi terrane and Calliope Volcanic Assemblage, New England Fold Belt, eastern Australia, indicate that the setting in which these rocks formed changed in both space and time. The Upper Silurian to Middle Devonian basalts of the Gamilaroi terrane show flat to slightly light rare‐earth element (LREE) depleted chondrite normalised patterns, depletion of high field strength elements (HFSE) relative to N‐MORB, low Ti/V and high Ti/Zr ratios, high Ni, Cr and large‐ion lithophile element (LILE) contents, features characteristic of intra‐oceanic island arc basaltic magmas. They are associated with low‐K, less mafic volcanics, showing moderate LREE enrichment, low Nb and Y contents and Rb/Zr ratios. The depletion of HFSE in the basalts indicates that the magmas were derived from a refractory source in a supra‐subduction zone setting. The presence of such a zone implies that the arc was associated with a backarc basin, the location of which was to the west where a wide backarc region existed from the Middle Silurian. This polarity of arc and backarc basin suggests that the subduction zone dipped to the west. In contrast to their older counterparts, Middle to Upper Devonian basalts of the Gamilaroi terrane have MORB‐like chondrite normalised patterns and higher Ti and lower LILE contents. Moreover, they have low Ti/Zr ratios and MORB‐like Ti/V ratios and HFSE contents, features typical of backarc basins. Dolerites of the Gamilaroi terrane also have predominantly backarc basin signatures. These features suggest that both the basalts and dolerites have been emplaced in an extensional environment produced during the rifting of the intra‐oceanic island arc lithosphere. A progressive increase in Ti/V ratios, and TiO₂ and Fe₂O₃ contents at constant MgO, of stratigraphically equivalent basalts, towards the north‐northwest part of the belt, is consistent with either greater extension to the north or melting of a more fertile magma source. By contrast, basalts in the southeast part of the terrane have moderately high Ti/Zr and low Ti/V ratios and in some samples, exhibit depletion of HFSE, compositional features transitional between island arc and backarc basin basalts. The Lower to Middle Devonian mafic rocks in the Calliope Volcanic Assemblage show both LREE enriched and depleted chondrite normalised REE patterns. Further, the majority have high Ti/Zr ratios and low Zr contents as well as relatively high Th contents relative to MORB. These features are common to rocks of Middle Devonian age as well as those of Early Devonian age and are suggestive of eruption in an arc setting. Thus, the data from this study provide new evidence for the evolution of the New England Fold Belt from the Late Silurian to the Late Devonian and reveal a history more complicated than previously reported.     

北祁连山扎麻什地区东沟蛇绿岩LA-ICP-MS锆石U-Pb测年及其地球化学特征

东沟蛇绿岩位于北祁连造山带中东段的扎麻什一带,主要由辉橄岩、辉长岩和基性火山岩组成较为完整的蛇绿岩单元。对基性火山岩进行单颗粒锆石LA-ICP-MS U-Pb同位素测定,获得499.3Ma±6.2Ma年龄加权平均值,代表蛇绿岩的形成年龄,相当于晚寒武世。岩石地球化学研究表明,该蛇绿岩中的基性火山岩属于拉斑玄武岩系列,球粒陨石标准化稀土元素分配模式为近平坦型,(La/Yb)N在0.97～1.26之间;微量元素分配模式除个别大离子亲石元素(Ba、Rb、U、K)外基本为平坦型曲线,Nb、Ta、Zr、Hf无亏损,显示出洋中脊玄武岩(N-MORB)的地球化学特征;在Zr-Zr/Y和Ti/100-Zr-3Y等构造环境判别图中,所有样品数据点均落入MORB区域内,表明其形成于洋中脊环境。经区域对比,该蛇绿岩与玉石沟、川刺沟蛇绿岩等一起构成了大洋扩张脊型蛇绿岩带。     

西藏泽当—罗布莎蛇绿岩的地球化学特征及其构造意义

西藏泽当—罗布莎蛇绿岩是雅鲁藏布江蛇绿岩带的重要组成部分,受控于板块碰撞结合带。该蛇绿岩主要由变质橄榄岩、辉长辉绿岩及玄武岩等组成。变质橄榄岩具有低的A l2O3和CaO含量,以富Mg,贫Ti、∑REE为特征,与世界上典型蛇绿岩中方辉橄榄岩的特征值一致。辉长岩和玄武岩的主量元素、微量元素特征显示其含有洋脊拉斑玄武岩和大洋岛弧拉斑玄武岩的双重成分,其中高场强元素Nb、Ta、Zr、H f等亏损,大离子亲石元素Rb、Sr、Ba等相对富集,具有大洋岛弧玄武岩的特点。辉长岩在球粒陨石标准化稀土元素配分模式图上为LREE亏损的平坦型,无负Eu异常,与洋中脊玄武岩类似。根据上述特征并结合区域地质构造特征,认为泽当?罗布莎蛇绿岩可能形成于边缘海盆地的海底慢速扩张环境。     

Geochemistry of Proterozoic volcanic and granitic rocks from the Gold Hill-Wheeler Peak area,northern New Mexico

Early Proterozoic supracrustal and plutonic rocks from the Gold Hill-Wheeler Peak area in northern New Mexico define three populations: amphibolite—diorite—tonalite, hornblendite—cumulus amphibolite and felsic volcanics and porphyries. Also present are mid-Proterozoic granites. Amphibolites are similar in Ti, Zr, Cr, Ni and REE contents to young calc-alkaline and arc basalts and diorites and tonalites are similar in composition to young andesites and to high-Al₂O₃ tonalites, respectively. Felsic volcanics resemble young felsic volcanics from mature arc systems in their immobile-element contents. Geochemical model studies suggest that the amphibolites, hornblendites, diorites and tonalites are related by progressive fractional crystallization of a hydrous parent tholeiite magma produced from partial melting of undepleted lherzolite. Amphibolites represent parent tholeiites modified by olivine removal. Hornblendite is an early solid residue comprised chiefly of hornblende, clinopyroxene, and olivine; diorite and cumulus amphibolite represent respectively residual solid (clinopyroxene, plagioclase, hornblende) and liquid, after 50% crystallization. Tonalite represents a residual liquid after 80% crystallization. Felsic volcanic rocks are produced by partial melting of a tonalite or diorite source with granulite-facies mineralogy in the lower crust. Granites have a similar origin to felsic volcanics although requiring an inhomogeneous source with the presence of residual hornblende or garnet.The calc-alkaline igneous rocks in the Gold Hill-Wheeler Peak area suggest the presence of an arc system in northern New Mexico during the Early Proterozoic. The fact that these rocks interfinger with and are overlain by mature clastic sediments favors a model in which a continental arc system is uplited, eroded and buried by cratonic sediments from the north.     

Neoproterozoic zirconium-depleted boninite and tholeiitic series rocks from Adola, southern Ethiopia

In Adola, southern Ethiopia, mafic and ultramafic igneous rocks occur in narrow, 4–10 km wide, north-south-trending belts bounded by high-grade gneisses and migmatites. The mafic/ultramafic rocks are complexly deformed and metamorphosed in greenschist to lower amphibolite facies and are thought to be tectonically dismembered parts of an ophiolite complex. Preliminary geochemical and geochronological data highlight that the high-grade rocks in southern Ethiopia and northern Kenya include a significant portion of juvenile rocks that were accreted at the same time as ophiolitic rocks at 885-765 Ma. This is also the time of widespread oceanic magmatism and closure in the Arabian-Nubian Shield to the north.The Adola mafic rocks were previously described as island arc tholeiites and mid-ocean ridge basalts (MORB). New chemical analyses on the Megado belt rocks reveal the presence of boninites and related dacites interspersed with tholeiitic rocks. The Adola boninites are similar to the Cambrian boninites in western Tasmania in having relatively low Zr/Sm (≤32). Boninites with similarly low ratios have not been reported from elsewhere.The Adola tholeiites have high Ti/Zr (150–300). Mixing between tholeiite and boninite magmas may have resulted in elevated Ti/Zr (80–126) in some Adola boninites. Otherwise, Ti/Zr in the latter is low (20–40). Low Ti/Zr is characteristic of Tertiary boninites in the west Pacific. The fact that both Ti/Eu and Zr/Sm increase from the Adola and Tasmania type to the Tertiary boninites at constant Ti/Zr suggests that Ti might be an element that is also metasomatically added to the source of boninites and raises doubts about the role of amphibole in boninite petrogenesis.     

Geology and geochemistry of the Neoproterozoic Tuludimtu Ophiolite suite,western Ethiopia

The Kemashi Domain, a lithotectonic subdivision of the Neoproterozoic Tuludimtu Orogenic Belt of western Ethiopia, consists of a suite of mafic–ultramafic volcanic and plutonic rocks, and interbedded deep marine sediments, mainly graphite-bearing pelitic schists and phyllites, and graphitic quartzites and cherts. Pillow structures indicate submarine extrusion of the volcanics, whilst partings within some of the basalts may represent sheeted dykes. An associated mélange unit, composed of blocks of the same rock types as above, set in a fine schistose matrix, also occurs. This assemblage is interpreted as a dismembered ophiolite—the Tuludimtu Ophiolite—formed in a deep oceanic environment. A turbiditic sequence is also present in the domain.The Tuludimtu Ophiolite underwent intense compression during the Neoproterozoic Pan African Orogeny, resulting in early recumbent folding and westwards-directed thrusting, followed by reactivation of steeper zones of the thrusts as N–S orogen-parallel strike-slip shear zones, accompanied by refolding of early folds into upright horizontal folds. This was followed by development of deep crustal NNW–SSE orogen-transecting shear zones, which were reactivated as brittle faults during orogenic collapse of the Tuludimtu Belt. Metamorphism to lower greenschist facies grade accompanied orogenesis.Major, trace and REE geochemistry of volcanic and some plutonic igneous rocks, has been employed to define the tectonic setting of the terrane. Tectonic discrimination diagrams, utilising REE and HFSE, indicate a wide distribution spectrum but with the majority of samples plotting in arc basalt and MORB fields, suggesting derivation from sources similar to N-MORB and depleted MORB (typical of many arc basalts). Most of the samples exhibit a slight depletion of immobile elements, relative to N-MORB values and also show depletion of Zr, Ti, Nb and Y, implying that their source had been depleted by an earlier melting episode. Overall, the geochemistry typifies spreading centre basalts with some compositional features transitional to those of arc basalts, a characteristic of back-arc basalts.Lithological association, structural style and geochemistry of the rock assemblage in the Kemashi Domain thus define an ophiolite interpreted to have formed within a deep marine environment. This is thought to have been due to rifting of continental crust within a back-arc basin regime in a continental margin type extensional setting. Comparison with other ophiolitic terranes within the Arabian Nubian Shield, suggests that many of these terranes may represent back-arc basin type tectonic settings, similar to the Kemashi Domain. This supports the multi-stage accretion model for closure of the Mozambique Ocean, for which the Pacific Ocean may be a present day analogue.     

Petrology,geochemistry and tectonic setting of the Khoy ophiolite,northwest Iran: implications for Tethyan tectonics

The Khoy ophiolite in northwestern Iran represents a remnant of oceanic lithosphere formed in the Mesozoic Neo-Tethys. This northwest–southeast trending ophiolite complex consists from bottom to top (east to west) of a well-defined basal metamorphic zone, peridotites (dunite, harzburgite) and serpentinized peridotite, gabbros, sheeted dikes, pillow and massive lava flows, and pelagic sedimentary rocks, including radiolarian chert. The rocks of the metamorphic zone have an inverse thermal gradient from amphibolite facies to greenschist facies. The high-grade metamorphic rocks are immediately adjacent to the peridotite and the gabbros and the low-grade rocks are in contact with the Precambrian Kahar Formation. Based on mantle-normalized incompatible trace element diagrams there are two distinct types of basalt flows present at the Khoy ophiolite: (1) massive basalts that have patterns virtually identical to E-MORB, and (2) pillow basalts that have more primitive chemical composition whose trace element patterns plot between E-MORB and N-MORB. The chondrite-normalized REE patterns for the pillow basalts are LREE-depleted [(La_N/Sm_N)_ave=0.70], similar to patterns for the mean diabase composition for the Oman ophiolite and LREE-depleted basalts of the Band-e-Zeyarat ophiolite of southern Iran. The REE patterns for the massive basalts are similar in general REE abundances to the pillow basalt patterns, but they are slightly LREE-enriched [(La_N/Sm_N)_ave=1.09] and their patterns cross those of the pillow basalts. The REE patterns for the gabbros and diorites indicates that the crustal-suite rocks were most likely derived by a process of fractional crystallization from a common basaltic melt. This basaltic melt was most likely generated by approx. 20–25% partial melting of a simple lherzolite source and had REE concentrations of roughly 10× chondrite. A comparison between the results from the Khoy ophiolite and the data from other Iranian ophiolites reveals geochemical evidence to suggest a tectonic link between the Khoy ophiolite and the rest of the Iranian ophiolites. Our results suggest that Khoy ophiolite is equivalent to the inner group of Iranian ophiolites (e.g. Nain, Shahr-Babak, Sabzevar, Tchehel Kureh and Band-e-Zeyarat) and was formed as a result of closure of the northwestern branch of a narrow Mesozoic seaway which once surrounded the Central Iranian microcontinent.     

甘蒙北山地区奥陶纪火山岩地球化学特征及形成环境探讨

对甘蒙（甘肃-内蒙古）北山地区花牛山、洗肠井、伊哈托里及希热哈达出露的奥陶纪火山岩进行了地球化学研究。结果表明:该区火山岩类型主要为安山玄武岩,以拉斑系列为主。花牛山和洗肠井火山岩的稀土元素配分具有弱的轻稀土亏损-平坦、重稀土略富集的特征,高场强元素比值特征与蛇绿岩（N-MORB）一致,形成环境为洋中脊;伊哈托里和希热哈达火山岩具轻稀土富集,高场强元素Nb、Ta、Ti等亏损的特征,类似于岛弧火山岩,但其Th_N/Nb_N(>>1.00)、Nb/La(<1.00)、Zr/Nb值类似于N-MORB,暗示火山岩遭受了地壳物质的混染作用。伊哈托里和希热哈达火山岩样品w(Zr)>70.00×10^-6和Zr/Y>3.00,显示了板内玄武岩的特征。结合区域构造演化分析,认为花牛山和洗肠井火山岩形成于洋中脊环境,而伊哈托里和希热哈达火山岩应属伸展裂解环境的产物。     

西昆仑东段苏巴什洋向北俯冲:来自早—中二叠世火山岩的证据

西昆仑东段苏巴什蛇绿构造混杂岩带南侧卡拉勒塔什群以大面积分布的酸性和中基性火山岩为特征,本次对卡拉勒塔什群弧火山岩代表性的岩石组合进行了LA-ICP-MS锆石U-Pb年龄、地球化学及锆石Lu-Hf同位素研究。研究结果表明,LA-ICP-MS锆石U-Pb测年获得酸性晶屑凝灰岩、蚀变玄武岩²⁰⁶Pb/²³⁸U加权平均年龄为(284.2±1.6) Ma和(262.6±2.0) Ma,表明研究区卡拉勒塔什群火山岩形成于早—中二叠世。卡拉勒塔什群火山岩具有富铝、钠、铁,富集大离子亲石元素K、Rb、Ba和亏损高场强元素Sr、Ta、Nb、Ti的地球化学特征。其中,基性火山岩属钙碱性-拉斑玄武岩系列,岩石稀土元素配分模式接近大洋拉斑玄武岩,Nd/Th和La/Nb比值为8.91~13.76和0.39~2.28,Lu-Hf同位素ε_Hf(t)值为-0.15~4.95,表现为地幔物质来源,但加入了地壳组分。酸性火山岩属于钙碱性系列,相对亏损P和Zr元素,Nd/Th和La/Nb比值为1.92~4.10和2.52~3.39,Lu-Hf同位素ε_Hf(t)值分别为0.94~3.78和8.26~12.45,二阶段模式年龄分别为1.07~1.25 Ga、0.51~0.78 Ga,表明酸性火山岩物质来源为古老地壳和新生地壳物质重熔后的混合物。卡拉勒塔什群总体地球化学特征表现为岛弧环境。卡拉勒塔什群岛弧火山岩与北侧苏巴什蛇绿构造混杂岩带在形成时代、空间分布以及基性岩地球化学特征均表现成对关系,与苏巴什蛇绿构造混杂岩带内发育的硫磺达坂砂岩组深水复理石建造共同构成造山带沟-弧-盆体系,表明苏巴什洋盆由南向北的俯冲极性,说明苏巴什蛇绿构造混杂岩形成于岛弧偏向于海沟的弧前盆地构造背景。     

滇西吉义独蛇绿混杂岩的岩石地球化学特征、成因和构造环境探讨

滇西吉义独蛇绿混杂岩位于金沙江缝合带的南端,岩石组合出露较齐全,包括堆晶橄榄岩、堆晶辉石岩、堆晶辉长岩以及玄武岩等,它们呈构造岩片的形式产出并与外来岩块组成蛇绿混杂岩.堆晶橄榄岩和辉石岩具低Al_2O_3,低TiO_2,而高Mg~#值(Mg~#=0.88～0.92),富集Cr和Ni,稀土总量偏低(∑REE=14.82×10~(-6)～27.75×10~(-6)),倒"U"型的稀土元素分布特征.堆晶辉长岩和玄武岩的Mg~#值较低,分别为0.70～0.79和0.51～0.66,具拉斑系列的演化趋势.玄武岩可以细分为2组:第一组玄武岩以平坦型稀土配分模式,低Mg~#(Mg~#=0.44～0.46),低稀土总量(∑REE=52.29×10~(-6)～60.26×10~(-6))为特征;第二组玄武岩则为LREE弱富集型的稀土配分模式,其Mg~#较高(Mg~#=0.54～0.68),稀土总量也较高(∑REE=62.13×10~(-6)～101.87×10~(-6)).在原始地幔标准化的微量元素配分图解中,两组玄武岩均相对富集大离子亲石元素而亏损Nb、Ta和Ti,与岛弧岩浆岩类似,明显不同于N-MORB.岩石的Sr-Nd同位素组成较为均一和稳定,堆晶橄榄岩和辉石岩的(~(87)Sr/~(86)Sr)_i=0.7051～0.7056,5_(Nd)(t)=2.8～4.1,玄武岩的(~(87)Sr/~(86)Sr)_i=0.7050～0.7056,ε_(Nd)(t)=5.1～5.8,且显示出原始地幔的同位素组成特征,暗示这些岩石为同源岩浆分异演化而成的岩浆产物.岩浆演化的主要方式为分离结晶作用,受地壳混染不明显.岩浆结晶形成岩石的顺序为:堆晶橄榄岩→堆晶辉石岩→堆晶辉长岩→玄武岩2组→玄武岩1组.岩石地球化学特征表明,吉义独蛇绿岩的形成与俯冲作用有关,且形成于金沙江洋内俯冲的消减环境.     

西藏日喀则白马让蛇绿岩:亚洲大陆边缘的小洋盆

藏南雅鲁藏布蛇绿岩被认为是新特提斯大洋岩石圈的残留。该带中段的日喀则白马让蛇绿岩是保存较完整的蛇绿岩岩块之一。该蛇绿岩主要由橄榄岩、蛇纹岩、镁铁质侵入岩和玄武岩组成,缺堆晶岩系。镁铁质侵入岩主要呈辉绿岩脉、岩床和少量的岩墙产出。辉绿岩脉在整个蛇绿岩层序中均有分布,侵入橄榄岩的部分岩脉已经变为变辉绿岩和异剥钙榴岩。辉绿岩床(墙)向上逐渐过渡为玄武岩。局部可见日喀则群整合覆盖在玄武岩之上。地球化学分析显示不同产状的镁铁质岩均属于低钾或中钾的拉斑玄武岩,亏损Nb、Ta、Ti和LREE,具有弧前玄武岩(FAB)或弧后盆地玄武岩(BABB)的特征,它们的Ti/V和Yb/V的比值与BABB或正常大洋中脊玄武岩(N-MORB)相似,Sr-Nd-Pb同位素数据指示了亏损地幔(DM)与富集地幔(EM)过渡的源区。镁铁质岩野外产出关系和地球化学特征表明,白马让蛇绿岩的镁铁质岩组合可能形成于SSZ环境。考虑到超镁铁质岩、镁铁质岩和日喀则群在空间上的连续性,认为白马让蛇绿岩可能是起源于亚洲大陆边缘俯冲带上的洋盆,属于原地系统,而非外来的构造岩片。     

The tectonic-setting of ophiolites in the western Qinghai-Tibet Plateau,China

The study of geology, geochemistry, rare earth elements, trace elements, Pb and Sr isotopes of representative ophiolite bodies from four ophiolitic belts in the western Qinghai-Tibetan Plateau, shows that the mantle peridotites of these ophiolites are mainly harzburgite in composition, with minor dunite. They are characterized by high magnesium (MgO) and low aluminum, calcium and alkali oxide contents. Enrichment of light rare earth elements in mantle peridotites may be due to two geological processes: relatively strong partial melting; and later metasomatism by the liquids released during the subduction of oceanic crust. Mantle peridotites are characterized by low contents of the trace elements Sr, Ti and Y and relatively high contents of Rb, Nb, Zr, Hf and Th, similar to metasomatic pyrolite. The isotopic compositions of Sr and Pb show evidence of contamination by a crustal component. All the evidence indicates that the four ophiolite belts in the western Qinghai-Tibetean Plateau have undergone metasomatism by liquids released during the subduction of oceanic crust, suggesting that they were formed in a supra-subduction zone (SSZ) tectonic setting. The mantle peridotites in ophiolite belts located in eastern Qinghai-Tibetan Plateau, e.g. Sanjiang and West Kunlun, may be compared with the Troodos, which is regarded as a typical SSZ complex, having the same geochemical characteristics, i.e. high MgO and LREE-rich. The geochemistry, combined with the occurrence of boninite and adakite rocks, which are associated with subduction magmatism, suggest that ophiolites formed at different times in Qinghai-Tibetan Plateau, including Sanjiang and West Kunlun, are all SSZ-type ophiolites formed in a supra-subduction zone tectonic setting.     

滇东南八布蛇绿岩地球化学特征及构造背景

八布蛇绿岩出露在北西向文山-麻栗坡和富宁走滑断裂之间的断块内,主要由蛇纹岩、辉长岩、玄武岩三个单元组成,彼此间均以断裂相接.地球化学研究表明,蛇纹岩SiO2、Al2O3、TiO2、MgO含量变化较大,依据主、微量元素组成可以分为两类,第一类球粒陨石标准化稀土元素分配模式整体较为平坦,推测原岩为镁铁质堆晶岩;另一类呈U型配分,推断原岩是方辉橄榄岩.玄武岩属拉斑系列,主量元素具有富MgO、TiO2,低Al2O3、K2O、P2O5,并且Na2O﹥K2O等特征,类似于MORB型玄武岩;REE配分模式也显示出N-MORB型玄武岩的特征;同时,微量元素普遍具有LILE、Th富集, Nb、Ta、Zr、Hf等元素具明显亏损特征,又暗示源区曾遭受不同程度的俯冲带流体交代作用.综合分析认为,八布蛇绿岩形成于弧后盆地环境,其形成可能是古太平洋从SE至NW向华南大陆俯冲的产物.     

西藏白朗地区蛇绿岩火山岩中单斜辉石的化学特征

近年来,应用岩石的地球化学和矿物学特征区分不同构造位置的现代火山岩,已被人们所注意。同时,根据这种观测资料所确定的一般特征,亦已广泛地应用于古火山岩,并获得了一些古地质环境的概念。