Effect of metamorphism and partial melting of host rocks on zircons

Abstract Zircons have been studied from different layers of migmatites (from Arvika, western Sweden and Nelaug, southern Norway) and from a paragneiss (from Arvika) associated with one of the migmatites. The main purpose of the investigation is to establish whether or not information about zircons can help in the elucidation of the parentage and rock-forming processes of migmatites.
The elongation ratio of zircons from all layers is small and characteristic of sedimentary zircons. Further, the absence of characteristic colours and the growth trends of the zircons (indicated by the reduced major axes) observed in the various samples both support a sedimentary parentage for these rocks. The zircons of all layers exhibit secondary growth (overgrowth, outgrowth and multiple growth) due to metamorphism. Compared with the zircons from the paragneiss, those of the migmatite layers are more clouded and less rounded, some of them becoming opaque or even skeletal; this is especially true of the zircons from the leucosomes. These observations indicate an alteration of the original sedimentary zircons in the migmatite, especially in the leucosomes, in response to the migmatization process, previously interpreted as partial melting.     

Petrogenesis of Adakitic Intrusive Rocksin Xu-Huai Area, Eastern China

1IntroductionAdakite has specially sense in the resuming structure background of rock formed,because of its distinctive geochemistry feature and implying partic-ular petrogenesis and ore genesis mechanism (De-fant and Drummond,1990),as a result,it arouses numerous researchers widely concern. After adakite conception was drawn by Chinese (Wang etal.,2000), it was widely concerned by domestic re-searchers (Wang etal.,2000,2001a,2001b;Xu etal.,2000;Pan etal.,2001;Qian,2001;Xu etal.,2001;X…     

Partial crustal melting beneath the Betic Cordillera (SE Spain): The case study of Mar Menor volcanic suite

The Neogene Volcanic Province (NVP) within the Betic Cordillera (SE Spain) consists of three main metapelitic enclave suites (from SW to NE: El Hoyazo, Mazarrón and Mar Menor). Since the NVP represents a singular place in the world where crustal enclaves were immediately quenched after melting, their microstructures provide a “photograph” of the conditions at depth just after the moment of the melting.

The thermobarometric information provided by the different microstructural assemblages has been integrated with the geophysical and geodynamical published data into a model of the petrologic evolution of the Mar Menor enclaves. They were equilibrated at 2–3 kbar, 850–900 °C, and followed a sequence of heating melt producing reactions. A local cooling event evidenced by minor melt crystallization preceded the eruption.

The lower crustal studies presented in this work contribute to the knowledge of: (i) the partial melting event beneath the Mar Menor volcanic suite through a petrologic detailed study of the enclaves; (ii) how the microstructures of fast cooled anatectic rocks play an important role in tracing the magma evolution in a chamber up to the eruption, and how they can be used as pseudothermobarometers; (iii) the past and current evolution of the Alborán Domain (Betic Cordillera) and Mediterranean Sea, and how the base of a metapelitic crust has melted within an active geodynamic setting.     

Partial Melting and Counterclockwise P T Path of Subducted Oceanic Crust (Sierra del Convento Melange, Cuba)

Partial melting of subducted oceanic crust has been identifiedin the Sierra del Convento mélange (Cuba). This serpentinite-matrixmélange contains blocks of mid-ocean ridge basalt (MORB)-derivedplagioclase-lacking epidote ± garnet amphibolite intimatelyassociated with peraluminous trondhjemitic–tonalitic rocks.Field relations, major element bulk-rock compositions, mineralassemblages, peak metamorphic conditions (c. 750°C, 14–16kbar), experimental evidence, and theoretical phase relationssupport formation of the trondhjemitic–tonalitic rocksby wet melting of subducted amphibolites. Phase relations andmass-balance calculations indicate eutectic- and peritectic-likemelting reactions characterized by large stoichiometric coefficientsof reactant plagioclase and suggest that this phase was completelyconsumed upon melting. The magmatic assemblages of the trondhjemitic–tonaliticmelts, consisting of plagioclase, quartz, epidote, ±paragonite, ± pargasite, and ± kyanite, crystallizedat depth (14–15 kbar). The peraluminous composition ofthe melts is consistent with experimental evidence, explainsthe presence of magmatic paragonite and (relict) kyanite, andplaces important constraints on the interpretation of slab-derivedmagmatic rocks. Calculated P–T conditions indicate counterclockwiseP–T paths during exhumation, when retrograde blueschist-faciesoverprints, composed of combinations of omphacite, glaucophane,actinolite, tremolite, paragonite, lawsonite, albite, (clino)zoisite,chlorite, pumpellyite and phengite, were formed in the amphibolitesand trondhjemites. Partial melting of subducted oceanic crustin eastern Cuba is unique in the Caribbean realm and has importantconsequences for the plate-tectonic interpretation of the region,as it supports a scenario of onset of subduction of a youngoceanic lithosphere during the early Cretaceous (c. 120 Ma).The counterclockwise P–T paths were caused by ensuingexhumation during continued subduction. KEY WORDS: amphibolite; Cuba; exhumation; partial melting; trondhjemite; subduction     

苏北盘石山、练山地幔捕虏体的PGE地球化学

通过锍镍火试金预富集法,分析了位于郯庐断裂带东侧的盘石山、练山地幔橄榄岩包体中铂族元素(PGE)和Au含量.不同于部分熔融残留成因地幔橄榄岩中通常所观察到的负斜率型或平坦型的分布模式,这两地的地幔橄榄岩以Pt、Pd、Ru相对富集,Ir、Rh相对亏损的"燕子型"分布模式为特征.Pt、Pd等不相容元素富集说明上地幔除经历过早期的部分熔融外,还经历了后期富Pt、Pd的高熔/岩比的熔(流)体的层析分离交代作用影响.盘石山地幔橄榄岩的PGE总量比练山高,Os的含量也比原始地幔值高;而练山地幔橄榄岩的Os含量比原始地幔值低,说明交代作用带走了练山地幔橄榄岩中的Os,却没有很大改变盘石山地幔橄榄岩中的Os含量,这可能与交代熔(流)体含硫量饱和程度有关.Rh的负异常可能与部分熔融过程中熔体较低的fo2有关.     

新疆北部晚古生代以来中基性岩脉的年代学、岩石学、地球化学研究

新疆北部中基性岩脉K-Ar表观年龄为187～271Ma,岩性以辉长、辉绿岩以及闪长、闪长玢岩为主,属于亚碱性系列.主量元素、稀土元素以及微量元素分析表明,中基性岩脉经历了源区地壳物质的混合以及侵位过程中的分离结晶作用,并在区域分布特点上受部分熔融程度的影响,而且由于结晶分异和部分熔融的不同,还出现了中基性岩脉系列的成分变异.排除上述岩浆作用的干扰,有证据显示岩脉起源于亏损地幔,由于Nd同位素模式年龄tDM集中在363～769Ma,反映源区是一个古生代时期的新生岩石圈地幔,该地幔源区属于大洋岩石圈地幔.新疆北部广泛出露的中基性岩脉在时间和空间上具有多样性,但是在产状、岩性组合和同位素特征上具有相似的特点,指示研究区在古生代以来具有一个相对统一和完整的源区,推测这个源区与北疆地区古生代以来长期存在的残余洋盆及其相关岩石圈有关.     

地幔岩中流体包裹体研究

地幔岩石中的流体包裹体代表地幔流体的样品。地幔流体包裹体可以存在从地幔来的金刚石,地幔捕虏体和岩浆碳酸岩中。研究这些岩石和矿物中的流体包裹体可以得出其所代表的地幔流体的温度、压力、成分和同位素。我们目前见到的这三类地幔岩石的包裹体主要可在橄榄石、辉石、金刚石、方解石和磷灰石中见到。这些包裹体可以粗略地分为CO2包襄体和硅酸盐熔融体包裹体。又可细分为四类包裹体：（1）富碳酸盐的硅酸盐熔融包裹体。这种包裹体在金刚石、地幔岩捕虏体和岩浆碳酸盐岩中见到,它又可分为结晶质熔融包裹体和玻璃包裹体。（2）CO2包裹体。这种包裹体大多见于地幔捕虏体中,在金刚石和岩浆碳酸岩中也可见到。（3）含硫化物的包裹体。这种包裹体见于地幔捕虏体中,与纯CO2包裹体和含CO2的熔融包裹体共存。（4）高密度的流体包裹体。这种包裹体见于金刚石中,是一种高盐度、高密度的含K、Cl和H2O的流体包裹体,又可分为高卤水包裹体和含卤水的富硅的碳酸盐岩浆包裹体。从对金刚石、地幔捕虏体和岩浆碳酸盐岩中流体包裹体的研究表明,地幔流体存在不均匀性和不混溶性。     

Origin and evolution of alkali basaltic magma in the Okinawa Trough

I~IOXThe Okinawa Trough is an extending back--arc basin between the East China Sea Shelf andthe Ry'Ukyu Island Arc of Japan. There are widespreadly distributing acid pumice in the troughand a little basalt just in some area of the extending center. There have been some detailed rePOrtsabout the mineralogy and petrochemical feature of the subalkali tholeiite and alkali trachyte in thetrough (Zhai and Gan, 1995; Li et al., 1997; Qin and Zhai, 1988). This paper mainly reportselectron mic…     

黄河河流水体二氧化碳分压及其影响因素分析

根据2003年秋季黄河河流水体二氧化碳分压(p(CO2))的实测数据，结合水文、化学和生物等要素的同步观测资料，对河流水体p(CO2)的影响因素进行了探讨。研究结果表明：生物好氧呼吸作用不是影响河流水体p(CO2)的主要因素，黄河河流p(CO2)的高低主要受水体碳酸盐系统的控制。     

A map series of the Southern East Pacific Rise and its flanks, 15° S to 19° S

Four large-scale bathymetric maps of the Southern East Pacific Rise and its flanks between 15° S and 19° S display many of the unique features of this superfast spreading environment including abundant seamounts (the Rano Rahi Field), axial discontinuities, discontinuity migration, and abyssal hill variation. Along with a summary of the regional geology, these maps will provide a valuable reference for other sea-going programs on-and off-axis in this area, including the Mantle ELectromagnetic and Tomography (MELT) experiment.