Origin of the Zedang and Luobusa Ophiolites,Tibet

The Zedang and Luobusa ophiolites are located in the eastern section of the Yalung Zangbo ophiolite belt,and they share similar geological tectonic setting and age.Thus,an understanding of their origins is very important for discussion of the evolution of the Eastern Tethys Ocean.There is no complete ophiolite assemblage in the Zedang ophiolite.The Zedang ophiolite is mainly composed of mantle peridotite and a suite of volcanic rocks as well as siliceous rocks,with some blocks of olivinepyroxenite.The mantle peridotite mainly consists of Cpx-harzburgite,harzburgite,some lherzolite,and some dunite.A suite of volcanic rocks is mainly composed of caic-aikaline pyroclastic rocks and secondly of tholeiitic pillow lavas,basaltic andesites,and some boninitic rocks with a lower TiO2 content （TiO2 ＜ 0.6％）.The pyroclastic rocks have a LREE-enriched REE pattern and a LILE-enriched （compared to HFSE） spider diagram,demonstrating an island-arc origin.The tholeiitic volcanic rock has a LREE-depleted REE pattern and a LILE-depleted （compared to HFSE） spider diagram,indicative of an origin from MORB.The boninitic rock was generated from fore-arc extension.The Luobusa ophiolite consists of mantle peridotite and mafic-ultramaflc cumulate units,without dike swarms and volcanic rocks.The mantle peridotite mainly consists of dunite,harzburgite with low-Opx （Opx ＜ 25％）,and harzburgite （Opx ＞ 25％）,which can be divided into two facies belts.The upper is a dunite-harzburgite （Opx ＜ 25％） belt,containing many dunite lenses and a large-scale chromite deposit with high Cr203; the lower is a harzburgite （Opx ＞25％） belt with small amounts of dunite and lherzolite.The Luobusa mantle peridotite exhibits a distinctive vertical zonation of partial melting with high melting in the upper unit and low melting in the lower.Many mantle peridotites are highly depleted,with a characteristic U-shaped REE pattern peculiar to fore-arc peridotite.The Luobusa cumulates are composed of wehrlite and olivine-pyroxenite,of the P-P-G ophiolite series.This study indicates that the Luobusa ophiolite was formed in a fore-arc basin environment on the basis of the occurrence of highly depleted mantle peridotite,a high-Cr2O3 chromite deposit,and cumulates of the P-P-G ophiolite series.We conclude that the evolution of the Eastern Tethys Ocean involved three stages：the initial ocean stage （formation of MORB volcanic rock and dikes）,the forearc extension stage （formation of high-Cr203 chromite deposits and P-P-G cumulates）,and the islandarc stage （formation of caic-alkaline pyroclastic rocks）.     

西藏雅鲁藏布江缝合带西段东波蛇绿岩的构造背景特征

西藏东波蛇绿岩位于雅鲁藏布江缝合带西段,由地幔橄榄岩、辉石岩和辉长岩等组成。地幔橄榄岩主要为方辉橄榄岩、纯橄岩和少量二辉橄榄岩。岩体的边界出露玄武岩和硅质岩等。地幔橄榄岩中有少量辉石岩和辉长岩的脉岩,宽约1 m,走向北西,与岩体的构造线方向基本一致。各岩相岩石地球化学研究结果表明,东波蛇绿岩的岩相存在较大的差异,玄武岩具有与洋岛玄武岩(OIB)相似的地球化学特征,而地幔橄榄岩中辉石岩、辉长岩脉与洋中脊玄武岩(MORB)相似,形成于洋中脊环境,并受后期俯冲流体作用的改造。东波岩体中二辉橄榄岩具有与深海地幔橄榄岩较一致的轻稀土亏损特征,而方辉橄榄岩和纯橄岩的地球化学特征显示出岩体形成于MOR环境,后受到SSZ环境的改造。东波蛇绿岩的岩石地球化学特征显示其洋中脊叠加洋岛的构造背景。     

Supra-subduction zone magmatism of the Neyriz ophiolite,Iran: constraints from geochemistry and Sr-Nd-Pb isotopes

The Neyriz ophiolite along the northeast flank of the Zagros fold-thrust belt in southern Iran is an excellent example of a Late Cretaceous supra-subduction zone (SSZ)-related ophiolite on the north side of the Neotethys. The ophiolite comprises a mantle sequence including lherzolite, harzburgite, diabasic dikes, and cumulate to mylonitic gabbro lenses, and a crustal sequence comprising a sheeted dike complex and pillow lavas associated with pelagic limestone and radiolarite. Mantle harzburgites contain less CaO and Al₂O₃, are depleted in rare earth elements, and contain spinels that are more Cr-rich than lherzolites. Mineral compositions of peridotites are similar to those of both abyssal and SSZ- peridotites. Neyriz gabbroic rocks show boninitic (SSZ-related) affinities, while crustal rocks are similar to early arc tholeiites. Mineral compositions of gabbroic rocks resemble those of SSZ-related cumulates such as high forsterite olivine, anorthite-rich plagioclase, and high-Mg# clinopyroxene. Initial εNd(t) values range from +7.9 to +9.3 for the Neyriz magmatic rocks. Samples with radiogenic Nd overlap with least radiogenic mid-ocean ridge basalts and with Semail and other Late Cretaceous Tethyan ophiolitic rocks. Initial ⁸⁷Sr/⁸⁶Sr ranges from 0.7033 to 0.7044, suggesting modification due to seafloor alteration. Most Neyriz magmatic rocks are characterized by less radiogenic ²⁰⁷Pb/²⁰⁴Pb (near the northern hemisphere reference line), suggesting less involvement of sediments in their mantle source. Our results for Neyriz ophiolite and the similarity to other Iranian Zagros ophiolites support a subduction initiation setting for its generation.     

西藏吉定蛇绿岩地球化学特征及其构造指示意义

吉定蛇绿岩位于雅鲁藏布江蛇绿岩带的中段,是该带保存较好的蛇绿岩之一,通过对该岩体的研究及与附近蛇绿岩剖面的对比有助于恢复早白垩世雅鲁藏布江蛇绿岩带的演化过程。吉定蛇绿岩包括玄武岩、辉绿岩、堆晶岩及地幔橄榄岩四个岩石单元。壳层岩石岩浆结晶顺序为:橄榄石→单斜辉石→斜长石,代表湿岩浆系统分异。吉定蛇绿岩壳层熔岩(玄武岩和辉绿岩)Ti O2含量为0.87%~1.45%,平均1.1%,与印度洋N-MORB玻璃(1.19%)相似。REE配分模式具有明显的LREE亏损特征,稀土配分模式与典型的大洋中脊玄武岩相似。但其微量元素蛛网图上表现为富集LILE,而亏损HFSE,并具有较高LILE/HFSE比值特征,与俯冲带上的(SSZ)蛇绿岩相似。蛇绿岩熔岩在岩石地球化学上表现出既亲MORB,又具部分IAB的特征。结合区域上大竹卡、得几等蛇绿岩岩石及地球化学资料对比分析,提出吉定蛇绿岩形成于在洋内俯冲带上发育起来的弧后盆地,并提出日喀则地区早白垩世洋壳演化的解释模式:雅鲁藏布江中段蛇绿岩至少包含三种组分特征的蛇绿岩体,其代表性剖面分别是吉定,得村和大竹卡,分别形成于近俯冲带的弧后盆地、弧前盆地和弧后盆地,这些洋壳共同组成早白垩世时期的与特提斯洋俯冲带斜交的一条分段发育的洋中脊。     

Platinum-group elemental geochemistry of mafic and ultramafic rocks from the Xigaze ophiolite, southern Tibet

The Xigaze ophiolite in the central part of the Yarlung–Zangbo suture zone, southern Tibet, has a well-preserved sequence of sheeted dykes, basalts, cumulates and mantle peridotites at Jiding and Luqu. Both the basalts and diabases at Jiding have similar compositions with SiO₂ ranging from 45.9 to 53.5 wt%, MgO from 3.1 to 6.8 wt% and TiO₂ from 0.87 to 1.21 wt%. Their Mg^#s [100Mg/(Mg + Fe)] range from 40 to 60, indicating crystallization from relatively evolved magmas. They have LREE-depleted, chondrite-normalized REE diagrams, suggesting a depleted mantle source. These basaltic rocks have slightly negative Nb- and Ti-anomalies, suggesting that the Xigaze ophiolite represents a fragment of mature MORB lithosphere modified in a suprasubduction zone environment. The mantle peridotites at Luqu are high depleted with low CaO (0.3–1.2 wt%) and Al₂O₃ (0.04–0.42 wt%). They display V-shaped, chondrite-normalized REE patterns with (La/Gd)_N ratios ranging from 3.17 to 64.6 and (Gd/Yb)_N from 0.02 to 0.20, features reflecting secondary metasomatism by melts derived from the underlying subducted slab. Thus, the geochemistry of both the basaltic rocks and mantle peridotites suggests that the Xigaze ophiolite formed in a suprasubduction zone.Both the diabases and basalts have Pd/Ir ratios ranging from 7 to 77, similar to MORB. However, they have very low PGE abundances, closely approximating the predicted concentration in a silicate melt that has fully equilibrated with a fractionated immiscible sulfide melt, indicating that the rocks originated from magmas that were S-saturated before eruption. Moderate degrees of partial melting and early precipitation of PGE alloys explain their high Pd/Ir ratios and negative Pt-anomalies. The mantle peridotites contain variable amounts of Pd (5.99–13.5 ppb) and Pt (7.92–20.5 ppb), and have a relatively narrow range of Ir (3.47–5.01 ppb). In the mantle-normalized Ni, PGE, Au and Cu diagram, they are relatively rich in Pd and depleted in Cu. There is a positive correlation between CaO and Pd. The Pd enrichment is possibly due to secondary enrichment by metasomatism. Al₂O₃ and Hf do not correlate with Ir, but show positive variations with Pt, Pd and Au, indicating that some noble metals can be enriched by metasomatic fluids or melts carrying a little Al and Hf. We propose a model in which the low PGE contents and high Pd/Ir ratios of the basaltic rocks reflect precipitation of sulfides and moderate degrees of partial melting. The high Pd mantle peridotites of Xigaze ophiolites were formed by secondary metasomatism by a boninitic melt above a subduction zone.     

罗布莎蛇绿岩地幔橄榄岩同位素特征及其成因

罗布莎蛇绿岩岩石单元出露齐全,以地幔橄榄岩为主,构造变形强烈;在堆积杂岩中上部产出有仰冲型斜长花岗岩小岩块;辉绿岩以脉状形式产出于地幔橄榄岩的方辉橄榄岩中;壳层岩石相对很薄且比较复杂.Sr、Nd和Pb同位素组成研究表明:罗布莎地幔橄榄岩绝大多数样品具有高ISr值(0.705489～0.714625);低INd值(0.5...     

Clinopyroxene REE Geochemistry of the Red Hills Peridotite, New Zealand: Interpretation of Magmatic Processes in the Upper Mantle and in the Moho Transition Zone

The Red Hills peridotite in the Dun Mountain ophiolite of SouthIsland, New Zealand, is assumed to have been produced in a paleo-mid-oceanridge tectonic setting. The peridotite is composed mostly ofharzburgite and dunite, which represent residual mantle andthe Moho transition zone (MTZ), respectively. Dunite channelswithin harzburgite blocks of various scales represent the MTZcomponent. Plagioclase- and clinopyroxene-bearing dunites occursporadically within common dunites. These dunites representproducts of melt–wall-rock interaction. Chondrite-normalizedrare earth element (REE) patterns of MTZ clinopyroxenes showa wide compositional range. Clinopyroxenes in plagioclase dunitesare extremely depleted in light REE (LREE) ([Lu/La]_N >100),and are comparable with clinopyroxenes in abyssal peridotitesfrom normal mid-ocean ridges. Interstitial clinopyroxenes inthe common dunite have flatter patterns ([Lu/La]_N 2) comparablewith those for dunite in the Oman ophiolite. Clinopyroxenesin the lower part of the residual mantle harzburgites are evenmore strongly depleted in LREE ([Lu/La]_N = 100–1000) thanare mid-ocean ridge peridotites, and rival the most depletedabyssal clinopyroxenes reported from the Bouvet hotspot. Incontrast, those in the uppermost residual mantle harzburgiteand harzburgite blocks in the MTZ are less LREE depleted ([Lu/La]_N= 10–100), and are similar to those in plagioclase dunite.Clinopyroxenes in the clinopyroxene dunite in the MTZ are similarto those reported from mid-ocean ridge basalt (MORB) cumulates,and clinopyroxenes in the gabbroic rocks have compositions similarto those reported from MORB. Strong LREE and middle REE (MREE)depletion in clinopyroxenes in the harzburgite suggests thatthe harzburgites are residues of two-stage fractional melting,which operated initially in the garnet field, and subsequentlycontinued in the spinel lherzolite field. The early stage meltingproduced the depleted harzburgite. The later stage melting wasresponsible for the gabbroic rocks and dunite. Strongly LREE–MREE-depletedclinopyroxene in the lower harzburgite and HREE-enriched clinopyroxenein the upper harzburgite and plagioclase dunite were formedby later reactive melt migration occurring in the harzburgite. KEY WORDS: clinopyroxene REE geochemistry; Dun Mountain ophiolite; Moho transition zone; orogenic peridotite; Red Hills     

Geochemical constraints on the origin of the Hegenshan Ophiolite,Inner Mongolia,China

The Hegenshan ophiolite in Inner Mongolia is a remnant of oceanic lithosphere of probable Devonian age. The ophiolite consists of several blocks composed chiefly of serpentinized ultramafic rocks with lesser amounts of troctolite and gabbro, and sparse lavas and dikes. The ultramafic rocks consist chiefly of depleted harzburgite and minor dunite and are interpreted as mantle tectonites. In the Hegenshan block dunite is relatively abundant and is typically associated with podiform chromitite. Both the chromite ore and the residual chromites in this body are relatively aluminous with average Cr numbers of 44–54. A few small chromite bodies and some of the residual chromites have much higher Cr numbers (72–76). Several blocks have well-layered cumulate sequences of gabbro and troctolite. Sheeted dikes are absent but small mafic dikes are common in some of the ultramafic sections. Most of the mafic dikes have flat chondrite-normalized REE patterns and are strongly depleted in incompatible elements, similar to depleted tholeiites from immature island arcs. The basaltic lavas of the Hegenshan ophiolite have two distinctly different chemical signatures—one similar to the mafic dikes and one similar to ocean island basalts. The entire complex was probably formed within an island arc–marginal basin system that was later accreted to the southern margin of the Siberian Altaids.     

The Eastern Makran Ophiolite (SE Iran): evidence for a Late Cretaceous fore-arc oceanic crust

ABSTRACT

The nature, magmatic evolution, and geodynamic setting of both inner and outer Makran ophiolites, in SE Iran, are enigmatic. Here, we report mineral chemistry, whole-rock geochemistry, and Sr–Nd–Pb isotope composition of mantle peridotites and igneous rocks from the Eastern Makran Ophiolite (EMO) to assess the origin and tectono-magmatic evolution of the Makran oceanic realm. The EMO includes mantle peridotites (both harzburgites and impregnated lherzolites), isotropic gabbros, diabase dikes, and basaltic to andesitic pillow and massive lava flows. The Late Cretaceous pelagic limestones are found as covers of lava flows and/or interlayers between them. All ophiolite components are somehow sheared and fragmented, probably in Cenozoic time, during the emplacement of ophiolite. This event has produced a considerable extent of tectonic melange. Tectonic slices of trachy-basaltic lavas with oceanic island basalt (OIB)-like signature seal the tectonic melange. Our new geochemical data indicate a magmatic evolution from fore-arc basalt (FAB) to island-arc tholeiite (IAT)-like signatures for the Late Cretaceous EMO lavas. EMO extrusive rocks have high εNd(t) (+8 to +8.9) and isotopically are similar to the Oman lavas. This isotopic signature indicates a depleted mid-ocean ridge basalt (MORB) mantle source for the genesis of these rocks, except isotopic gabbros containing lower εNd(t) (+5.1 to +5.7) and thus show higher contribution of subducted slab components in their mantle source. High ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb isotopic ratios for the EMO igneous rocks also suggest considerable involvement of slab-derived components into the mantle source of these rocks. The variable geochemical signatures of the EMO lavas are mostly similar to Zagros and Oman ophiolite magmatic rocks, although the Pb isotopic composition shows similarity to the isotopic characteristic of inner Zagros ophiolite belt. This study postulates that the EMO formed during the early stages of Neo-Tethyan subduction initiation beneath the Lut block in a proto-forearc basin. We suggest subduction initiation caused asthenospheric upwelling and thereafter melting to generate the MORB-like melts. This event left the harzburgitic residues and the MORB-like melts interacted with the surrounding peridotites to generate the impregnated lherzolites, which are quite abundant in the EMO. Therefore, these lherzolites formed due to the refertilization of mantle rocks through porous flows of MORB-like melts. The inception of subduction caused mantle wedge to be enriched slightly by the slab components. Melting of these metasomatized mantle generated isotropic gabbros and basaltic to andesitic lavas with FAB-like signature. At the later stage, higher contribution of the slab-derived components into the overlying mantle wedge causes formation of diabase dikes with supra-subduction zone – or IAT-like signatures. Trachy-basalts were probably the result of late-stage magmatism fed by the melts originated from an OIB source asthenospheric mantle due to slab break-off. This occurred after emplacement of EMO and the formation of tectonic melange.     

Supra-subduction and abyssal mantle peridotites of the Coast Range ophiolite,California

The Coast Range ophiolite (CRO) of California is one of the most extensive tracts of oceanic crust preserved in the North American Cordillera, but its origin has long remained controversial. We present here new data on mineral compositions in mantle peridotites that underlie crustal sections of the ophiolite, and show that these are dominantly refractory harzburgites related to high apparent melting in a supra-subduction zone (SSZ) setting. Abyssal peridotite (characterized by high-Al spinels and relatively high Ti, Na, Nd, Sm, Lu, and Hf in pyroxene) occurs at one location where it is associated with SSZ mantle peridotite and volcanic rocks with both oceanic and arc-like geochemistry. SSZ mantle peridotites (characterized by intermediate-Cr/Al or high-Cr spinels, and by extremely low Ti, Na, Nd, Sm, Lu, and Hf in pyroxenes) are associated with crustal sections containing arc-related volcanic rocks, including boninites. This convergence between conclusions based on crustal lithologies and their underlying mantle sections confirms previous proposals that link the CRO to SSZ processes, and seriously undermines hypotheses that invoke formation of the ophiolite at a mid-ocean ridge spreading center.     

内蒙古苏尼特左旗乌兰敖包基性-超基性岩的成因及其对含矿性的指示

乌兰敖包基性-超基性岩位于阿拉善北缘兴蒙造山带,由方辉橄榄岩、辉绿岩、辉长岩组成。地球化学分析表明:蛇纹石化橄榄岩为地幔橄榄岩,辉绿岩和辉长岩为拉斑玄武岩系列岩石,均具有与N-MORB相似的微量元素特征,但区别于N-MORB,3种岩石共同组成了蛇绿混杂岩。矿物学分析表明蛇纹石化橄榄岩中橄榄石为地幔橄榄岩中镁橄榄石(Fo=91.10~91.81),铬铁矿为铬尖晶石,具有高的Cr~#和Mg~#值分别为61.04~64.44、50.40~56.37)。单斜辉石出现在方辉橄榄岩、辉长岩及辉绿岩中,而斜方辉石只出现在方辉橄榄岩中,其中方辉橄榄岩中单斜辉石为顽透辉石,辉长岩中为普通辉石,而辉长岩中为次透辉石、贫钙普通辉石、普通辉石,斜方辉石均为斜顽辉石。辉长岩U-Pb年龄为344.5±1.5 Ma,表明该蛇绿混杂岩形成于早石炭世。地球化学及矿物学特征表明乌兰敖包蛇绿岩形成于俯冲早期的弧前环境中,属于SSZ型蛇绿岩。蛇绿混杂岩成因提供铬铁矿形成的有利条件,因此应围绕铬铁矿进行找矿工作。     

Seamount volcanism associated with the Xigaze ophiolite, Southern Tibet

Basaltic lavas at Renbu, Southern Tibet are associated with the Xigaze ophiolite in the Yarlung-Zangbo suture zone. They are alkaline lavas rich in large ion lithophile elements (LILE, Ba, Rb and Sr) and high field strength elements (HFSE, Nb, Ta, Zr and Hf), but poor in Cr, Co and Ni. All of the rocks have chondrite-normalized REE patterns enriched in light rare earth elements (LREE), comparable to modern basalts of the Society Islands, Kerguelen Plateau and Broken Ridge. Abundances of some immobile or moderately immobile elements (Nb, Ta, Zr, Hf, Y, Ti and REE) are also comparable to Kerguelen alkaline basalts. The Renbu basalts are geochemically similar to oceanic island basalts (OIB) and have some elemental ratios, such as Nb/Ta ratios = 15.7–18.1, Th/Nb =  0.06–0.10, La/Nb = 0.59–0.83 and Th/Ta = 1.03–1.52, similar to the primitive mantle. Their ⁸⁷Sr/⁸⁶Sr ratios (0.70453–0.70602) are relatively high, similar to OIB. In the ⁸⁷Sr/⁸⁶Sr vs. εNd(t) diagram, the Renbu basalts plot along a trend from N-MORB to EMII (enriched mantle II), suggesting the involvement of at least two mantle sources in their generation. The Renbu basalts represent seamount volcanism associated with the Xigaze ophiolite. They formed from an OIB-type mantle source within the Neo-Tethyan Ocean that had a composition similar to the modern Indian Ocean mantle.     

雅鲁藏布江缝合带西段达机翁地幔橄榄岩及铬铁矿的铂族元素特征

达机翁蛇绿岩位于雅鲁藏布江缝合带的西段北亚带,该蛇绿岩主要由地幔橄榄岩、玄武岩以及硅质岩组成,其中地幔橄榄岩以方辉橄榄岩为主,同时含有少量的纯橄榄岩,纯橄岩主要呈不规则透镜状或团块状分布于方辉橄榄岩中。在达机翁地幔橄榄岩中产出有3种不同类型的铬铁矿,分别为块状,豆状以及浸染状铬铁矿。文章主要对达机翁地幔橄榄岩的方辉橄榄岩及豆荚状铬铁矿进行了研究,结合岩石的主量元素和铂族元素,对地幔橄榄岩和豆荚状铬铁矿的成因以及雅鲁藏布江缝合带的找矿远景进行了探讨。达机翁地幔橄榄岩具有较高的Mg O含量以及较低的Al2O3和Ca O等含量,这种亏损的全岩成分指示了达机翁地幔橄榄岩经历了较高的部分熔融作用,同时方辉橄榄岩的PGEs的总量为23.68×10-9~31.02×10-9,高于原始地幔的值,Pd和Cu 2个元素的含量较为分散明显偏离部分熔融曲线,指示了达机翁方辉橄榄岩可能遭受了熔体的改造,在熔体-岩石反应的过程中,导致了富含PPGE的硫化物的加入。达机翁豆荚状铬铁矿为高Cr型铬铁矿,具有IPGE和Rh明显富集以及Pt,Pd明显亏损的特征,不同类型的铬铁矿之间具有一致的PGEs的分配模式。雅鲁藏布江缝合带内大量分布的超镁铁岩体在岩石组合、地球化学特征、成因以及形成时代等方面,均具有相似性,是中国铬铁矿找矿的有利远景区。     

The Stonyford Volcanic Complex: a Forearc Seamount in the Northern California Coast Ranges

Jurassic age volcanic rocks of the Stonyford volcanic complex(SFVC) comprise three distinct petrological groups based ontheir whole-rock geochemistry: (1) oceanic tholeiites; (2) transitionalalkali basalts and glasses; (3) high-Al, low-Ti tholeiites.Major and trace element, and Sr–Nd–Pb isotopic dataindicate that the oceanic tholeiites formed as low-degree partialmelts of normal mid-ocean ridge basalt (N-MORB)-source asthenospheresimilar in isotope composition to the East Pacific Rise today;the alkalic lavas were derived from an enriched source similarto that of E-MORB. The high-Al, low-Ti lavas resemble second-stagemelts of a depleted MORB-source asthenosphere that formed bymelting spinel lherzolite at low pressures. Trace element systematicsof the high-Al, low-Ti basalts show the influence of an enrichedcomponent, which overprints generally depleted trace elementcharacteristics. Tectonic discrimination diagrams show thatthe oceanic tholeiite and alkali suites are similar to present-daybasalts generated at mid-oceanic ridges. The high-Al, low-Tisuite resembles primitive arc basalts with an enriched, alkalibasalt-like overprint. Isotopic data show the influence of recycledcomponents in all three suites. The SFVC was constructed ona substrate of normal Coast Range ophiolite in an extensionalforearc setting. The close juxtaposition of the MORB-like olivinetholeiites with alkali and high-Al, low-Ti basalts suggestsderivation from a hybrid mantle source region that includedMORB-source asthenosphere, enriched oceanic asthenosphere, andthe depleted supra-subduction zone mantle wedge. We proposethat the SFVC formed in response to collision of a mid-oceanridge spreading center with the Coast Range ophiolite subductionzone. Formation of a slab window beneath the forearc duringcollision allowed the influx of ridge-derived magmas or themantle source of these magmas. Continued melting of the previouslydepleted mantle wedge above the now defunct subduction zoneproduced strongly depleted high-Al, low-Ti basalts that werepartially fertilized with enriched, alkali basalt-type meltsand slab-derived fluids. KEY WORDS: CRO; oceanic basalts; California     

新疆西准噶尔达拉布特蛇绿岩E-MORB型镁铁质岩的地球化学、年代学及其地质意义

新疆西准噶尔地区是古生代经过俯冲-增生形成的复合造山带,该地区分布有多条蛇绿岩带,其中之一的西准噶尔达拉布特蛇绿岩被认为是最大的一条蛇绿岩带,可能代表了古亚洲洋壳的残余。本文的资料显示蛇绿岩带内的镁铁质岩呈现出N-MORB、E-MORB和似OIB的地球化学特征,通过对阿克巴斯套岩体中的浅色辉长岩LA-ICP-MS锆石年龄测定,获得达拉布特蛇绿岩E-MORB型镁铁质岩的年龄为302±1.7Ma。鉴于达拉布特蛇绿岩中E-MORB和似OIB型镁铁质岩成因的复杂性,结合前人研究成果,对辉长岩锆石U-Pb年龄所代表的意义存在两种可能性：（1）E-MORB型和似OIB型镁铁质岩可能是弧后盆地扩张后期的产物,代表蛇绿岩的年龄,其表明西准噶尔地区可能晚石炭纪还有洋盆存在;（2）E-MORB型镁铁质岩是蛇绿岩消亡阶段由于扩张脊和俯冲带碰撞作用而形成的弧前海山,形成时代晚于达拉布特主体蛇绿岩,但其成因与蛇绿岩的演化密切相关。本文侵向于第二种可能性,认为新疆北部晚石炭-早二叠可能仍存在活动陆缘,俯冲作用仍然存在,扩张脊俯冲形成的板片窗效应导致地幔楔、俯冲板片和沉积物等熔融促使基性岩浆向长英质酸性岩浆转变,从而引发了二叠纪大规模玄武质岩浆底侵,导致了该时期的构造-岩浆-成矿-造山作用的发生。     

西昆仑库地蛇绿岩地质、地球化学及其成因研究

西昆仑库地蛇绿混杂岩由方辉橄榄岩和纯橄榄岩等地幔变质橄榄岩、豆荚状铬铁矿、堆晶橄榄岩、堆晶辉石岩和辉长岩、辉绿岩墙、块状和枕状玄武岩等组成。强亏损方辉橄榄岩为主的地幔岩组合,二辉石的低Al含量和铬尖晶石的高Cr^#,以及岩石的富Mg、Ni和贫Al、Ca特征一致表明地幔橄榄岩类是经较高程度部分熔融后的地幔残余,与消减带之上蛇绿岩中的同类岩石相近。岩石富Rb、Ba、U、Th、LREE,说明地幔残余岩石受到了来自消减带的洋壳重熔组分的混染。堆晶岩以辉石岩和辉长岩为主,可能属PPG系列,指示岩浆是在消减带环境和含水条件下熔融的。辉长岩为低Ti蛇绿岩型,代表洋内弧后盆地早期环境或弧前环境。辉绿岩和玄武岩为洋中脊拉斑玄武岩和岛弧拉斑玄武岩的过渡类型;玄武岩和辉绿岩相比富Ba、Th、LREE,贫Ta,指示玄武岩较辉绿岩更多地受到来自消减带洋壳重熔组分的影响。库地蛇绿岩形成时的古构造环境是消减带之上的弧间或弧后盆地。     

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。     

Petrogenetic evolution of the Koziakas ophiolite complex (W. Thessaly, Greece)

Summary The Koziakas ophiolitic complex is situated in W. Thessaly and is interpreted as an incomplete ophiolite sequence, thrust over the Western Thessaly Unit (W.T.U.). Two tectonically distinct units represent it: (1) the lower unit with a metamorphic sole and a tectonosedimentary mélange and (2) the upper unit including mantle peridotites and basaltic lavas. The mantle peridotites are composed by harzburgite, lherzolite and plagioclase lherzolite intruded by a sparse network of gabbroic, plagiogranitic and doleritic dykes. The volcanic sequence of the upper unit can be geochemically subdivided into four groups of basalts with: (1) tholeiitic N-type MORB affinities, (2) low-Ti boninitic affinities, (3) subalkalic E-MORB type affinities and (4) alkali characteristics displaying a different petrogenetic evolution with respect to the other groups. The magmatic history of the Koziakas ophiolite is in agreement with extensive fractional crystallization and variable degrees of partial melting of a heterogeneous mantle source, yielding, magmas mainly of MORB composition. Modal and cryptic metasomatic phenomena of the mantle peridotites as well as Sr-Nd isotopic ratios imply that this melt contained also a hydrous component derived from melting of a subducted lithosphere. The above geochemical characteristics and the correlation with the adjacent ophiolite suites of Pindos, Othris and Vourinos indicate that Koziakas ophiolitic complex formed in a small backarc basin situated at the eastern margin of the greater Pindos Ocean, between the Western Thessaly Unit (W.T.U.) and the Pelagonian continent.     

西藏雅鲁藏布江缝合带东段泽当地幔橄榄岩特征及其意义

泽当岩体位于雅鲁藏布江缝合带东段,主要由地幔橄榄岩、辉长辉绿岩和基性火山岩等组成。地幔橄榄岩主要为方辉橄榄岩和二辉橄榄岩,有少量透镜状纯橄岩。地幔橄榄岩经历了强烈的塑性变形作用。地幔橄榄岩中橄榄石的Fo值为89.6~91.8,属镁橄榄石;斜方辉石为顽火辉石,En 87.8~90.3;单斜辉石En 44.1~50.0,主要为顽透辉石和透辉石。铬尖晶石的Cr#值(=100×Cr/(Cr+Al))为17.0~93.6,其中,二辉橄榄岩和方辉橄榄岩中的铬尖晶石为富铝型尖晶石,纯橄岩中的铬尖晶石Cr#最高,为富铬型尖晶石。地幔橄榄岩的部分熔融程度为17%~34%,表明泽当地幔橄榄岩可能经历了多阶段的过程。亏损的主量元素组成和低于原始地幔的稀土元素含量(0.15×10-6~0.61×10-6)指示泽当地幔橄榄岩为经历过部分熔融和熔体抽取的亏损残余地幔岩石。REE配分型式为中稀土亏损的"V"型或"U"型,原始地幔标准化元素比值(La/Sm)N为0.5~8.0,表明泽当地幔橄榄岩经历过交代作用。矿物化学与地球化学数据表明泽当地幔橄榄岩形成于MOR环境,后受到SSZ环境的改造。     

Fossil seamount in southeast Zagros records intraoceanic arc to back-arc transition: New constraints for the evolution of the Neotethys

The Neotethys ocean is transiently involved in two subduction zones during the Late Cretaceous. While the Northern Neotethys subduction zone (below Eurasia) was active from the early Mesozoic until the Eocene, the intra-oceanic Southern Neotethys subduction zone only developed during the Late Cretaceous. We herein document, through a combination of structural, geochemical and geochronological data, the magmatic evolution of a Late Cretaceous supra-subduction ophiolite fragment of the Neotethys (the Siah Kuh massif, Southern Iran), now sandwiched in the Zagros suture zone. Results show that this ophiolite fragment — a subducted yet exceptionally well-preserved seamount — records an evolution from supra-subduction zone magmatism (including island arc tholeiites, boninites and calc-alkaline transitional magmatism) around 87 Ma, to MORB (from E-MORB to N-MORB) magmatism at 78 Ma, and potentially until 73 Ma. We conclude that this seamount initially formed in an arc context and represents either (i) a non-obducted remnant of the Oman ophiolite that experienced a longer-lived magmatic history (prefered hypothesis) or (ii) a piece from the forearc/frontal arc of the Northern margin of the Neotethys. Regardless of its exact original location, the Siah Kuh seamount was later subducted in the Northern Neotethys subduction zone.