Petrology and Geochemistry of the Dangqiong Ophiolite,Western Yarlung-Zangbo Suture Zone,Tibet,China

The Dangqiong ophiolite, the largest in the western segment of the Yarlung-Zangbo Suture Zone(YZSZ)ophiolite belt in southern Tibet, consists of discontinuous mantle peridotite and intrusive mafic rocks. The former is composed dominantly of harzburgite, with minor dunite, locally lherzolite and some dunite containing lenses and veins of chromitite. The latter, mafic dykes(gabbro and diabase dykes), occur mainly in the southern part. This study carried out geochemical analysis on both rocks. The results show that the mantle peridotite has Fo values in olivine from 89.92 to 91.63 and is characterized by low aluminum contents(1.5–4.66 wt%) and high Mg# values(91.06–94.53) of clinopyroxene. Most spinels in the Dangqiong peridotites have typical Mg# values ranging from 61.07 to 72.52, with corresponding Cr# values ranging from 17.67 to 31.66, and have TiO2 contents from 0 to 0.09%, indicating only a low degree of partial melting(10–15%). The olivine-spinel equilibrium and spinel chemistry of the Dangqiong peridotites suggest that they originated deeper mantle(20 kbar). The gabbro dykes show N-MORB-type patterns of REE and trace elements. The presence of amphibole in the Dangqiong gabbro suggests the late-stage alteration of subduction-derived fluids. All the lherzolites and harzburgites in Dangqiong have similar distribution patterns of REE and trace elements, the mineral chemistry in the harzburgites and lherzolites indicates compositions similar to those of abyssal and forearc peridotites, suggesting that the ophiolite in Dangqiong formed in a MOR environment and then was modified by late-stage melts and fluids in a suprasubduction zone(SSZ) setting. This formation process is consistent with that of the Luobusa ophiolite in the eastern Yarlung-Zangbo Suture Zone and Purang ophiolite in the western Yarlung-Zangbo Suture Zone.     

Petrology of the dykes from the Waziristan Ophiolite,NW Pakistan

The Waziristan Ophiolite is located in the suture zone between the Indian Plate to the east and Afghan Block to the west. It is highly dismembered and divisible into three main sheets or nappes, which from east to west are: the Vezhda Sar Nappe, entirely comprised of pillow basalts; the Boya Nappe, made up of ophiolitic melange with an intact section in its basal part; and the Datta Khel Nappe, consisting mainly of sheeted dykes with smaller proportions of other components. Faunal evidence suggests that the ophiolite is of Tithonian-Valanginian age. It was thrust over the Mesozoic shelf-slope sediments of the Indian Plate to the east during the Paleocene and is unconformably overlain by sedimentary rocks of Early to Middle Eocene age to the west. Beside the sheeted dykes, best exposed in the hanging wall of the Datta Khel Thrust ENE of Datta Khel, the ophiolite also contains isolated dykes. These are doleritic and basaltic in composition. The dykes contain high Na₂O contents and high FeO^t/MgO and LILE/HFSE ratios, and low TiO₂ (<0.1 wt%) and K₂O contents. Non-depletion of Nb and high LILE/HFSE ratio negate, respectively, an island-arc or mid-ocean ridge setting for these dykes. Enrichment in the LILE suggests the involvement of a crustal component driven by fluids along the subduction zone. Several geochemical parameters suggest that the dykes of Waziristan Ophiolite have transitional characteristics between mid-ocean ridge basalt and island-arc tholeiite. It is therefore proposed that these dykes may have originated in a back-arc basin tectonic setting.     

Preservation of biosignatures in metaglassy volcanic rocks from the Jormua ophiolite complex,Finland

Evidence of microbially-mediated alteration of basaltic glass is preserved in originally glassy basalts (rims of pillow lavas, hyaloclastite breccias, and chilled margins of dykes) from the well-preserved 1.95 Ga Jormua ophiolite complex (JOC) of Northeastern Finland. Although textural evidence of microbial alteration is commonly observed in relic glass from recent oceanic crust and some ophiolites, these textures have been destroyed during greenschist to lower amphibolite facies regional metamorphism and deformation of the JOC. However, another robust biosignature is found in the generally depleted δ¹³C values of disseminated carbonate extracted from originally glassy basalts, relative to crystalline samples. The same distribution of δ¹³C values is well documented in samples from recent oceanic crust as well as ophiolites of Phanerozoic age. This characteristic contrast in the δ¹³C values of disseminated carbonate is interpreted to result from microbe-induced fractionation during oxidation of organic matter. X-ray mapping of initial alteration zones has identified residual carbon associated with highly-concentrated S that is unrelated to carbonate. We attribute these biosignatures to microbially-mediated alteration of originally glassy material prior to ophiolite emplacement.     

Petrology and Geochemistry of the Late Eocene Harrison Pass Pluton, Ruby Mountains Core Complex, Northeastern Nevada

The late Eocene Harrison Pass pluton was emplaced in the transitionzone between the infrastructure and suprastructure of the RubyMountains core complex. Emplacement was at     

滇东北会泽灯影组硅质岩成因及沉积环境——来自岩石学和地球化学证据

通过1：50 000区域地质调查,发现云南省会泽县一带灯影组中普遍发育硅质岩。本文通过硅质岩岩石学特征和地球化学特征分析,重点探讨了灯影组各段硅质岩的成因机制和沉积环境。结果表明：震旦系-寒武系灯影组下段硅质岩中相对富集Al和Ti,总稀土（ΣREE）相对较高,Ce呈负异常,Eu呈正异常;中段硅质岩中相对富集Ca和Mg,总稀土较低,LREE>HREE,Ce呈负异常,Eu呈正异常;上段硅质岩总稀土较低,LREE>HREE,Ce大部分呈负异常,Eu呈正负异常。综合硅质岩岩石学、地球化学特征,并结合震旦纪区域地质构造演化,认为研究区内下段硅质岩表现为正常海水沉积、中段硅质岩表现为典型热水沉积、上段表现出正常海水沉积受热水沉积影响的特征。硅质岩形成于复杂大陆边缘的浅海陆棚环境。     

西藏班公湖-怒江缝合带中段江错蛇绿岩岩石学、地球化学、年代学及地质意义

江错蛇绿岩位于班公湖-怒江缝合带中段, 位于切里湖蛇绿岩亚带的最东端, 南邻蓬湖西蛇绿岩.江错蛇绿岩岩石组合相对较全, 主要由变质橄榄岩、辉长岩和辉绿岩等组成.江错蛇绿岩辉长岩-辉绿岩与N-MORB相比具有较高的Mg#、低Ti、K、Na和P的特征, 富集大离子亲石元素Sr、Rb、Ba和亏损高场强元素Th、Hf、Ta、Nb, REE配分图总体显示为平坦型分布模式.通过地球化学元素分析认为江错蛇绿岩是形成于SSZ之上的弧后盆地扩张脊环境.对其进行辉长岩锆石SHRIMP U-Pb定年, 得出加权平均年龄为189.8±3.3 Ma(MSWD=0.97), 该结果代表了班公湖怒江缝合带中段江错地区特提斯洋的扩张时代, 与中段东巧地区蛇绿岩年龄一致, 但晚于东段洋盆发育时代且早于西段洋盆发育时代, 表明整个班公湖-怒江洋盆发育时代存在东早西晚的特点.      

Petrology, Geochronology and Geochemistry of the early Cretaceous Ophiolite Complexes in Cebu Island, Central Philippines: A Clue to Mineralization

Please refer to the attachment(s) for more details     

Geochemistry and tectonic setting of mafic rocks from the Othris Ophiolite,Greece

We present new geochemical analyses of minerals and whole rocks for a suite of mafic rocks from the crustal section of the Othris Ophiolite in central Greece. The mafic rocks form three chemically distinct groups. Group 1 is characterized by N-MORB-type basalt and basaltic andesite with Na- and Ti-rich clinopyroxenes. These rocks show mild LREE depletion and no HFSE anomalies, consistent with moderate degrees (~15%) of anhydrous partial melting of depleted mantle followed by 30–50% crystal fractionation. Group 2 is represented by E-MORB-type basalt with clinopyroxenes with higher Ti contents than Group 1 basalts. Group 2 basalts also have higher concentrations of incompatible trace elements with slightly lower HREE contents than Group 1 basalts. These chemical features can be explained by ~10% partial melting of an enriched mantle source. Group 3 includes high MgO cumulates with Na- and Ti-poor clinopyroxene, forsteritic olivine, and Cr-rich spinel. The cumulates show strong depletion of HFSE, low HREE contents, and LREE enrichments. These rocks may have formed by olivine accumulation from boninitic magmas. The petrogenesis of the N-MORB-type basalts and basaltic andesites is in excellent agreement with the melting conditions inferred from the MOR-type peridotites in Othris. The occurrence of both N- and E-MORB-type lavas suggests that the mantle generating the lavas of the Othris Ophiolite must have been heterogeneous on a comparatively fine scale. Furthermore, the inferred parental magmas of the SSZ-type cumulates are broadly complementary to the SSZ-type peridotites found in Othris. These results suggest that the crustal section may be genetically related to the mantle section. In the Othris Ophiolite mafic rocks recording magmatic processes characteristic both of mid-ocean ridges and subduction zones occur within close spatial association. These observations are consistent with the formation of the Othris Ophiolite in the upper plate of a newly created intra-oceanic subduction zone. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Geochronology, Petrology and Geochemistry of Xingdi No. 3 Mafic-Ultramafic Intrusions in the Northeastern Tarim Craton, NW China

The Xingdi mafic-ultramafic intrusions occur in the northeastern margin of the Tarim craton. The Xingdi No. 3 intrusion is the smallest of four intrusions, with an exposed area of 1.7 km~2, and the zircon U-Pb age of the intrusion is 752±4 Ma. The intrusion consists of gabbros, pyroxenites and peridotites, and exhibits a crystallization sequence of the main rock-forming minerals as olivine, orthopyroxene, clinopyroxene and plagioclase. Mineralization occurred at or near the boundaries of the intrusion between pyroxenites and peridotites, and appears as a layered or lenticular shape about 500 m long and 4–15 m wide. The primary sulfides have a relatively simple mineralogy dominated by pyrrhotite-pentlandite-chalcopyrite assemblages, which occur as droplet, star-like and graphic texture and locally sideronitic structures. Geochronological and geochemistry investigations suggest that the Xingdi mafic-ultramafic intrusions and coeval volcanic rock in the Kuluktag area of the Tarim craton formed in an intracontinental breakup environment. Based on the composition of the dominant rockforming minerals and covariant relationships of other oxides versus Mg O, the parental magma of the Xingdi No.3 intrusion belongs to high-Mg tholeiitic basaltic magmas with Mg O of 10.78 wt%. The Xingdi No.3 intrusive rocks are characterized by light REE enrichment relative to heavy REE, negative Nb-Ta anomalies, low ~(143)Nd/~(144)Nd ratios(from 0.511183 to 0.511793) and high initial ~(87)Sr/~(86)Sr ratios(from 0.7051 to 0.7113). The magma was derived from the enriched-lithospheric mantle and was contaminated during emplacement. According to rock assemblages, mineralization, olivine characteristics, geochemical characteristics and mass balance, there are better copper-nickel ore prospects in the Xingdi No.3 intrusion than in the other three intrusions in the area.     

Petrology, geochemistry and paleogeographic reconstruction of the East Sulawesi Ophiolite, Indonesia

The East Sulawesi Ophiolite (ESO) is tectonically dismembered and widely distributed in Central and East Sulawesi. It comprises, from base to top, residual mantle peridotite and mafic–ultramafic cumulate through layered to isotropic gabbro, to sheeted dolerites and basaltic volcanic rocks. Residual peridotite is dominantly spinel lherzolite intercalated with harzburgite and dunite. Ultramafic rocks from different locations display significant differences in rock composition and mineral. However, the clinopyroxene of peridotite displays REE pattern similarities with those of mid-ocean ridge (MOR) origin, rather than those of suprasubduction zone (SSZ) origin. The gabbroic unit consists of massive gabbro, layered gabbro, mafic and ultramafic cumulate and anorthosite. The observed crystallization sequence of gabbroic unit, which is olivine→(spinel)→plagioclase→clinopyroxene→(orthopyroxene)→(hornblende), and the mineral chemistry data indicate that the ESO gabbro has similarities with MOR setting.Major and trace element geochemistry of basalt and dolerite suggests MOR, oceanic plateau and minor SSZ origins. A possible oceanic plateau origin is supported by the following: (i) the 15-km thickness is comparable with the thickness of oceanic plateau rather than normal oceanic lithosphere; (ii) there are no or only minor olivine phenocrysts in the basalt; and (iii) predominance of aphyric texture in the basalts. The REE pattern of ESO basalt exhibits N-MORB-like signatures. However, a negative Nb anomaly in the trace element spider diagram may be attributed to mantle heterogeneity of an OPB source.The geochemical variations and disparities for both peridotite and basalt and the noncogenetic relationship between crust and mantle sections in several locations suggest that the ESO may have been formed at one tectonic setting and was later overprinted by magmatism in different environments through its birth to emplacement. A possible Cretaceous origin of an oceanic plateau component of the ESO is indicated on the basis of calculated paleopositions using plate trajectory analyses together with previously published paleolatitude data. The ESO can be traced back to the proximity of the presently active region of the SW Pacific Superplume.     

新疆科克森套蛇绿岩套玄武岩的地质地球化学

新疆北部科克森套地区玄武岩出露于超镁铁质岩体的北侧,与玄武质凝灰岩共生,厚度达50～80 m;玄武岩普遍具有片理化、斑状结构;岩石地球化学特征表明,该玄武岩属低碱(Na2O+K2O均<3%,σ值在0.60～1.19)、低钛(TiO2在0.73%～1.22%)、富镁(MgO在7.43%～9.72%)的拉斑玄武岩系列,具有原始岩浆特征(其Mg#值在0.65～0.71),稀土总量较低(ΣREE在29.68×10-6～44.91×10-6),稀土及微量元素配分曲线平缓,可能形成于洋中脊构造环境。该区玄武岩的发现和岩石地球化学研究,佐证了科克森套蛇绿岩套的存在。     

赣东北蛇绿混杂岩岩石地球化学特征及构造意义

赣东北蛇绿混杂岩是中国为数不多的新元古代蛇绿岩,对该蛇绿混杂岩带中的方辉橄榄岩和玄武岩类岩石进行元素地球化学研究,以探讨新元古代赣东北地区的构造动力学背景.研究表明,方辉橄榄岩具有低TiO₂(0.02%~0.37%)、低K₂O(0~0.02%)、低Na₂O(0.02%~0.10%)和富MgO(40.81%~44.58%)特征,烧失量普遍较高(10.09%~13.47%),表明样品普遍遭受蚀变;稀土元素配分模式图和微量元素蛛网图显示,橄榄岩样品具有亏损地幔源区特征,同时兼有俯冲带流体交代的特征.玄武岩类SiO₂含量主要为52.63%~56.19%,平均为55.00%,属安山玄武岩范围.岩石具低MgO(3.66%~6.46%),中等TiO₂(1.14%~2.66%),高Al₂O₃(13.07%~15.27%)、Na₂O(3.42%~5.03%)的特征.安山玄武岩轻稀土富集((La/Yb)_N=1.84~2.61),无明显Eu异常,稀土配分曲线呈右倾型;微量元素蛛网图中显示大离子亲石元素Th、Ba、U、Pb富集、高场强元素Nb、Ti亏损,可能系俯冲板片脱水产生的流体交代地幔楔所形成,TNT(Ta、Nb和Ti)的负异常指示赣东北蛇绿混杂岩的形成过程中可能遭受了陆源物质的混染,反映了消减作用的影响.方辉橄榄岩和安山玄武岩的地球化学特征表明赣东北蛇绿混杂岩的形成可能经历了多期演化,早期可能系古华南洋发生洋内俯冲作用,俯冲过程中释放的流体交代了俯冲带上的地幔楔,使地幔橄榄岩富集轻稀土.随后的弧后盆地扩张阶段,形成具有岛弧性质和MORB特征的玄武岩类,但由于弧后盆地发育不成熟,玄武岩类在地球化学上显示有大陆物质的特征,表现出高Pb、低Ti和轻稀土强烈富集.      

昆中蛇绿岩岩石学和地球化学

作为昆南变质地体与柴达木-中阿尔金地块边界的中昆仑缝合带,沿带断续出露蛇绿杂岩.本次研究涉及布青山蛇绿岩、诺木洪蛇绿岩和阿其克库勒湖西缘蛇绿岩,通过对其岩石学、岩石化学和微量元素与稀土元素地球化学特征分析,确定其属蛇绿岩类,证明蛇绿杂岩带性质属板块缝合带.     

Geochemistry of peridotites and mafic igneous rocks from the Central Dinaric Ophiolite Belt,Yugoslavia

In the Central Dinaric Ophiolite Belt (CDOB) peridotites and associated metamorphic rocks of various grades tectonically overlie an olistostrome melange of middle to late Jurassic age. Peridotites and underlying slices of mafic granulites (partially transformed to gamet amphibolites) are intruded by doleritic dikes which do not occur in the melange. The melange contains blocks of subgreywackes and cherts as well as those of pillow lavas and massive diabase (spilites). CDOB peridotites are in the spinel peridotite facies, but locally spinel-plagioclase peridotites occur as well. All peridotites have lherzolitic compositions showing several significant element correlations: Al₂O₃, CaO, TiO₂, Na₂O and Cu are negatively correlated and Ni is positively correlated with MgO. Recent estimates of primitive mantle compositions lie near the low-MgO end point of each correlation trend. Al/Ti and Ca/Al ratios of CDOB lherzolites are for the most part higher than the range observed in chondrites. However, when a few samples with extreme compositions are excluded, Al/Ti and Ca/Al are positively correlated with MgO, and the samples at the low-MgO end have near-chondritic Ca/Al but slightly higher than chondritic Al/Ti ratios. Chondrite-normalized REE patterns of CDOB lherzolites show extreme depletions in LREE providing strong evidence for the absence of any metasomatic renrichment. The lack of correlation between highly incompatible elements (LREE) and moderately incompatible elements (HREE, Ti, Na, Al, Ca) together with the extremely low La/Sm ratios suggest that fractional or very small increment melt removal played a role in the genesis of these lherzolites. Four out of five lherzolites yield and apparent Sm-Nd isochron age of 136±15 Ma with an Nb of 6.0±1.1 (bulk rocks and clinopyroxene separates). One sample has an exceptionally high Nd of about 23. The mafic igneous rocks scatter around the lower end of the 136 Ma reference isochron allowing, but not proving, a genetic relationship with a mantle having a Nd isotopic composition which is similar to that of CDOB lherzolites. LIL element abundances of spilites and doleritic dike rocks suggest some hydrothermal alteration. In primitive mantle-normalized concentration diagrams none of these mafic igneous rocks shows a significant negative Nb-Ta anomaly. Chondrite-normalized REE patterns of both rock types are essentially flat. Whereas the inferred primary compositions of the spilites compare well with those of E-type MORBs, the doleritic dike rocks show elemental ratios similar to those normally found in back-arc basin tholeiites.     

Geochemistry, Petrology and Mineralogy of Coal Measure Shales in the Middle Jurassic Yanan Formation from Northeastern Ordos Basin, China: Implications for Shale Gas Accumulation

The Jurassic Yanan Formation is one of the most important coal-producing formations and hydrocarbon source rocks in the Ordos Basin, North China. To evaluate the shale gas potential of the Yanan shale, a total of 48 samples from north Ordos Basin were sampled, and their geochemical, petrological, mineralogical and pore characteristics were investigated. It was found that the shale samples are a suite of early mature source rock. The total organic carbon (TOC) content ranges from 0.33% to 24.12% and the hydrogen index (HI) ranges from 43.31mg/g to 330.58 mg/g. The relationship between Tmax and HI indicates the organic matter is type Ⅱ-Ⅲ. This conclusion is also supported by the organic petrological examination results, which shows that the kerogen is mainly liptinite and vitrinite. Minerals in the samples are composed mainly of quartz, clay and feldspar, and the clay minerals are composed of prevailing kaolinite, illite/smectite, chlorite and a small amount of illite. Under scanning electron microscope, OM pores in the Yanan shale are scarce except pores come from the kerogen intrinsic texture or clay aggregates within the organic particles. As the weak compaction caused by shallow burial depth, interparticle pores and intraparticle pores are common, the hydrocarbon storage capacity of the Yanan shale was improved. According to evaluation, the Yanan shale is considered as a good shale gas reservoir, but its hydrocarbon potential is more dependent on biogenic and coal-derived gas as the thermogenic gas is limited by the lower thermal maturity.     

Petrology and Geochemistry of Cretaceous Ultramafic Volcanics from Eastern Kamchatka

The origin, evolution and primary melt compositions of lateCretaceous high-K ultramafic volcanics and associated basaltsof Eastern Kamchatka are discussed on the basis of a study ofthe mineralogy and geochemistry of the rocks and magmatic inclusionsin phenocrysts. The exceptionally primitive composition of thephenocryst assemblage [olivine—Fo;_88–95, Cr-spinel—Cr/(Cr + Al) up to 85] provides direct evidence of the mantleorigin of primary melts, which were highly magnesian compositions(MgO 19–24 wt%). The rocks and meltsare characterizedby strong high field strength element (HFSE) depletion in comparisonwith rare earth elements, and high and variable levels of enrichmentin large ion lithophile elements (LILE), P, K and H₂O (0.6–12wt % in picritic to basaltic melts). _Nd values lie in a narrowrange (+107 to +91), typical of N-MORB (mid-ocean ridge basalt),but ⁸⁷Sr/⁸⁶Sr (0.70316–0.70358) is slightly displacedfrom the mantle array. High-K ultramafic melts from Kamchatkaare considered as a new magma type within the island-arc magmaticspectrum; basaltic members of the suite resemble arc shoshonites.The primary melts were produced under high-pressure (30–50kbar) and high-temperature(1500–1700C) conditions bypartial melting of a refractory peridotitic mantle. KEY WORDS: Kamchatka; Late Cretaceous magmatism; ultramafic volcanics; shoshonites ^*Corresponding author. Present address: Department of Geology, University of Tasmania, GPO Box 252C, Hobart, Tas., Australia     

Petrology and Geochemistry of Mantle Peridotite Xenoliths from SE China

Geochemical data on Type I spinel peridotite and garnet peridotitexenoliths in Cenozoic basalts from SE China demonstrate thatthe lithospheric mantle under this region is heterogeneous.The depletion and enrichment shown by these peridotite xenolithsare not related to their locations as suggested earlier. Samplesfrom individual localities, at the continental margin or thecontinental interior, show large variational ranges from depletedharzburgite to fertile Iherzolite. The measured Nd and Sr isotopiccompositions of clinopyroxene separates range from _Nd 49 to160 and from ⁸⁷Sr/⁸⁶Sr 070256 to 070407, respectively. Thedepleted signatures of Sr and Nd isotopic compositions and major-elementcontents (low CaO and Al₂O₃ in most xenoliths require an olddepletion event, probably mid-Proterozftic, and the enrichmentof LREE in the depleted peridotites implies a young metasomaticevent shortly before Cenozoic magmatism. Major-element compositionsof the peridotite xenoliths are controlled largely by the degreeof partial melting, and the extra fertile peridotites (highCaO and Al₂O₃) are probably the products of interaction betweenperidotites and a basaltic component. The equilibrium P–Tconditions, determined from coexisting mineral phases, indicatethat these xenoliths equilibrated over a wide P–T range,from 770 to 1250 C and from 10 to 27 kbar. Calculated oxygenfugacities for most spinel peridotites range from near the FMQbuffer to 25 log units below. The late-stage metasomatism didnot change the redox state in the upper mantle. ^*Corraponding author     

The Geochemistry and Experimental Petrology of Sodic Alkaline Basalts from Oatlands, Tasmania

Sodic basalts of Oligocene-Early Micene age occur within anEarly Tertiary graben in the Oatlands district of Tasmania.They include olivine tholeiites, alkali olivine basalts, basanites,transitional nephelinites, nepheline hawaiites, and nephelinemugearites. They have compositional characteristics in commonwith sodic alkaline basalt suites from other parts of the world.With decreasing SiO₂, concentrations of CaO, alkalis, P₂O₅,and incompatible trace elements increase. Compositional and experimental data for the basalts are consistentwith their derivation by polybaric partial melting of a garnetlherzolite source enriched in P₂O₅, light rare earths, Nb, andother incompatible trace elements. Experimental data for a primitivenepheline basanite from the Oatlands district indicate thatconcentrations of H₂O+CO₂ need not have been more than 6?5 wt.%for the original basanite magma to have derived from an amphibole-bearinggarnet lherzolite source. In the case of more SiO₂-undersaturatedolivine melilitites from the neighbouring Central Plateau, theexperimental evidence is consistent with either higher concentrationsof H₂O+CO₂ (approaching 14 wt.%), or higher pressures of origin(>35 kb). Petrographic and geochemical evidence suggeststhat the latter is the more probable of the alternatives.     

Petrology and Petrogenesis of Cumulate Peridotites and Gabbros from the Marum Ophiolite Complex, Northern Papua New Guinea

The Marum ophiolite complex in northern Papua New Guinea includesa thick (3–4 km) sequence of ultramafic and mafic cumulates,which are layered on a gross scale from dunite at the base upwardsthrough wehrlite, lherzolite, plagioclase lherzolite, pyroxenite,olivine norite-gabbro and norite-gabbro to anorthositic gabbroand ferrogabbro at the top. Igneous layering and structures,and cumulus textures indicate an origin by magmatic crystallizationin a large magma chamber(s) from magma(s) of evolving composition.Most rocks however show textural and mineralogical evidenceof subsolidus re-equilibration. The cumulate sequence is olivine and chrome spinel followedby clinopyroxene, orthopyroxene and plagioclase, and the layeredsequence is similar to that of the Troodos and Papuan ophiolites.These sequences differ from ophiolites such as Vourinos by thepresence of cumulus magnesian orthopyroxene, and are not consistentwith accumulation of low pressure liquidus phases of mid-oceanridge-type olivine tholeiite basalts. The cumulus phases show cryptic variation from Mg- and Ca-richearly cumulates to lower temperature end-members, e.g. olivineMg_93–78, plagioclase An_94–63. Co-existing pyroxenesdefine a high temperature solidus with a narrower miscibilitygap than that of pyroxenes from stratiform intrusions. Re-equilibratedpyroxene pairs define a low-temperature, subsolidus solvus.Various geothermometers and geobarometers, together with thermodynamiccalculations involving silica buffers, suggest the pyroxene-bearingcumulates crystallized at 1200 °C and 1–2 kb pressureunder low f_O2. The underlying dunites and chromitites crystallizedat higher temperature, 1300–1350 °C. The bulk of thecumulates have re-equilibrated under subsolidus conditions:co-existing pyroxenes record equilibration temperatures of 850–900°C whereas olivine-spinel and magnetite-ilmenite pairs indicatefinal equilibration at very low temperatures (600 °C). Magmas parental to the cumulate sequence are considered to havebeen of magnesian olivine-poor tholeiite composition (>50per cent SiO₂, 15 per cent MgO, 100 Mg/(Mg + Fe²⁺) 78) richin Ni and Cr, and poor in TiO₂ and alkalies. Fractionated examplesof this magma type occur at a number of other ophiolites withsimilar cumulate sequences. Experimental studies show that suchlavas may result from ial melting of depleted mantle lherzoliteat shallow depth. The tectonic environment in which the complexformed might have been either a mid-ocean ridge or a back-arebasin.