湖北枣阳大阜山镁铁/超镁铁杂岩体与金红石矿床成因

湖北省北部枣阳市境内的大阜山镁铁/超镁铁杂岩体主要由未变质的纯橄岩和橄长岩组成,伴生变质的石榴石角闪石岩、石榴石钠黝帘石角闪石岩和角闪石钠黝帘石岩.杂岩体的围岩为大理岩.该岩体中的金红石矿床产于变质的石榴石角闪石岩或蚀变的石榴石角闪石岩中.橄长岩中锆石SHRIMP U-Pb同位素定年结果表明镁铁/超镁铁质岩体的成岩年龄约为600Ma.变质的岩石和未变质的岩石几乎具有完全相同的地球化学特征,表明前者的原岩应为橄长岩,整个岩体是一个分异的镁铁/超镁铁侵入体.金红石赋存于角闪石解理中等岩相特征表明矿床的含矿岩石就是石榴石角闪石岩,不是由石榴石辉石岩退变质形成的;且其中的石榴石为钙铁榴石,不是富镁、富铝的与高压/超高压作用有关的石榴石.金红石矿床的形成和岩体的局部变质应该与由北向南的近水平推覆构造有关,动力变质作用提供的热和流体,结合推覆过程中的变形作用,导致围岩大理岩和镁铁质岩体发生交代作用,形成石榴石角闪石岩和金红石矿床.     

Petrogenesis of a high-temperature metamorphic terrane: a new tectonic interpretation for the north Dabieshan, central China

ABSTRACT The northern Dabie terrane consists of a variety of metamorphic rocks with minor mafic-ultramafic blocks, and abundant Jurassic-Cretaceous granitic plutons. The metamorphic rocks include orthogneisses, amphibolite, migmatitic gneiss with minor granulite and metasediments; no eclogite or other high-pressure metamorphic rocks have been found. Granulites of various compositions occur either as lenses, blocks or layers within clinopyroxene-bearing amphibolite or gneiss. The palaeosomes of most migmatitic gneisses contain clinopyroxene; melanosomes and leucosomes are intimately intermingled, tightly folded and may have formed in situ. The granulites formed at about 800–830 °C and 10–14 kbar and display near-isothermal decompression P–T paths that may have resulted from crust thickened by collision. Plagioclase-amphibole coronae around garnets and matrix PI + Hbl assemblages from mafic and ultramafic granulites formed at about 750–800 °C. Partial replacement of clinopyroxene by amphibole in gneiss marks amphibolite facies retrograde metamorphism. Amphibolite facies orthogneisses and interlayered amphibolites formed at 680–750 °C and c. 6 kbar. Formation of oligoclase + orthoclase antiperthite after plagioclase took place in migmatitic gneisses at T ≤ 490°C in response to a final stage of retrograde recrystallization. These P–T estimates indicate that the northern Dabie metamorphic granulite-amphibolite facies terrane formed in a metamorphic field gradient of 20–35 °C km^-1 at intermediate to low pressures, and may represent the Sino-Korean hangingwall during Triassic subduction for formation of the ultrahigh- and high-P units to the south. Post-collisional intrusion of a mafic-ultramafic cumulate complex occurred due to breakoff of the subducting slab.     

Petrology of mafic-ultramafic rocks along North Puruliya Shear zone,West Bengal

The North Puruliya Shear zone (NPSZ) is characterized by occurrence of mafic-ultramafic rocks aligned parallel to the shear zone, intruding the high grade Proterozoic rocks of Chhotanagpur Gneissic Complex. The ultramafic rocks occur as small lenses, pockets, veins, thin dykes and are intimately associated with mafic (gabbro, norite) rocks. Pyroxenites (viz. olivine websterite, websterite, plagioclase websterite) and hornblendite are the two important members of the ultramafic rocks containing clinopyroxene, orthopyroxene, olivine, plagioclase, amphibole, phlogopite and ilmenite. The mafic-ultramafic rocks show evidence of shearing and retrogressive metamorphism. Linear correlation of chemical attributes suggests fractionation-controlled magmatic differentiation. Enrichment of LILE and LREE in the mafic-ultramafic suite suggests an enriched mantle source and pronounced negative Eu-anomalies in all the rock types except hornblendite suggest fractionation of plagioclase under low f_O2 condition. Progressive iron enrichment trend in rocks of the mafic-ultramafic suite also indicate magmatic differentiation under low f_O2 condition. Early fractionation and accumulation of clinopyroxene and plagioclase from a basaltic magma may have given rise to the ultramafic rocks of the area. Little change in the Nb/Zr and Ce/Zr ratios of ultramafic and mafic rocks (except alkali norite) strongly support low crustal contamination. A few samples of norite and gabbro-norites appeared to be variably contaminated by a crustal component or affected by late granitic intrusion resulting in enrichment of alkali in the former.     

鄂西黄陵背斜南部元古宙庙湾蛇绿岩的发现及其构造意义

对鄂西黄陵背斜南部宜昌太平溪、邓村一带崆岭岩群中的元古宙庙湾岩组强烈变形变质超镁铁—镁铁质岩的研究表明,镁铁质岩主要为似层状细粒斜长角闪岩,变辉长岩岩体、岩脉及辉绿岩岩脉,超镁铁质岩则主要为蛇纹石化纯橄榄岩、方辉橄榄岩,呈构造岩片、岩块分布于斜长角闪岩之中。细粒斜长角闪岩TiO2=1.14%～1.48%,稀土元素配分型式为略亏损—平坦型,无明显的Eu异常,(La/Yb)N=0.87～1.12,La/Nb、Ce/Zr、Zr/Nb、Zr/Y、Ti/Y平均值分别为1.04、0.15、18.78、2.53、290.51,Nb/Th平均为9.88,显示为大洋中脊构造环境形成的N-MORB型拉斑玄武岩;变辉长岩具典型的堆晶结构特征,稀土元素配分型式为平坦型,具明显的Eu正异常;蛇纹石化纯橄榄岩的稀土元素配分型式具中稀土元素略亏损的U形特征,显示为LREE略富集的地幔岩。上述特征表明,黄陵背斜南部崆岭岩群中的元古宙庙湾岩组实际上是一套混杂堆积的古大洋蛇绿岩残片。元古宙庙湾蛇绿岩的发现为华南扬子克拉通存在中元古代洋盆和哥伦比亚超大陆聚合、裂解构造事件提供了重要的证据。     

磷灰石Eu/Y-Ce:基于大数据的源区类型判别新图解SCIEI北大核心CSCD

磷灰石广泛分布于火成岩、沉积岩和变质岩中,是一种常见的、包含丰富微量元素的副矿物。磷灰石晶格可容纳丰富的微量元素,且因其形成的物理化学条件不同会表现出差异明显的微量元素特征。利用磷灰石微量元素特征可以追踪物质来源和演化。现在常用的方法是利用磷灰石的微量元素绘制二元判别图解,经典判别图解包括Sr-Y、Sr-Mn、Y-(Eu/Eu^(*))和(Ce/Yb)_(N)-REE图解。随着微区测试技术发展,磷灰石微量元素数据日渐丰富,同时由于磷灰石化学成分的复杂性,传统图解已逐渐无法有效利用这些数据所携带的信息,进而无法准确判别其生成环境。建立能准确判别磷灰石物源的新型判别图解故而迫切。近年来,磷灰石微量元素数据的大量积累,为运用以大数据为依托,准确判别磷灰石物源奠定了数据基础。本研究将大数据技术与地球化学数据相结合,共收集整理了1925个代表性磷灰石测试点的微量元素数据,对富碱性火成岩、超镁铁质岩石、镁铁质火成岩、长英质花岗岩、中-低级变质岩、高级变质岩六种类型中磷灰石微量元素数据进行穷举端元处理,共获得7140个磷灰石物源判别图解端元组合,在轮廓系数限定下,进一步有效筛选并提取出能判别磷灰石物源类型的最优图解端元。本文构建了Eu/Y-Ce磷灰石判别新图解,相较于之前的磷灰石判别图解,其涵盖了更全面的物源类型,可以更准确地判别源区类型。     

Geochemical evolution of high-pressure mafic granulites from the Bacariza formation (Cabo Ortegal complex, NW Spain): an example of a heterogeneous lower crust

 The Cabo Ortegal complex (northwestern Iberian massif) is a klippen formed of several structural units stacked during the Hercynian collision. All these units include ultramafic rocks, metabasites and quartz-feldspathic gneisses affected by different metamorphic conditions. The Bacariza formation is heterogeneous showing a conspicuous layering mainly defined by alternate high-pressure ultrabasic-to-basic granulites, retrogressed garnet amphibolites of intermediate composition and rare acid rocks forming garnet trondhjemitic gneisses. This layering is inherited from a gabbroic protolith showing a composition rich in Fe and Ti. Major and trace elements of these rocks can be correlated to continental tholeiitic series of extensional settings. These high-pressure granulites are situated in normal contact between±serpentinised ultramafic rocks and other high-grade metabasites with lessevolved and more-depleted composition comparable to T-type and N-type MORB. It is suggested that the layered gabbro-type protolith was part of a continuous mafic crust. This crust was initially formed during Early Ordovician in a continental extensional setting and progressively evolved to oceanic spreading. Received: 9 February 1996/Accepted: 10 February 1997     

Ultrahigh-pressure metamorphism and exhumation of garnet-bearing ultramafic rocks from the Lanterman Range (northern Victoria Land, Antarctica)

Northern Victoria Land is a key area for the Ross Orogen – a Palaeozoic foldbelt formed at the palaeo‐Pacific margin of Gondwana. A narrow and discontinuous high‐ to ultrahigh‐pressure (UHP) belt, consisting of mafic and ultramafic rocks (including garnet‐bearing types) within a metasedimentary sequence of gneisses and quartzites, is exposed at the Lanterman Range (northern Victoria Land). Garnet‐bearing ultramafic rocks evolved through at least six metamorphic stages. Stage 1 is defined by medium‐grained garnet + olivine + low‐Al orthopyroxene + clinopyroxene, whereas finer‐grained garnet + olivine + orthopyroxene + clinopyroxene + amphibole constitutes the stage 2 assemblage. Stage 3 is defined by kelyphites of orthopyroxene + clinopyroxene + spinel ± amphibole around garnet. Porphyroblasts of amphibole replacing garnet and clinopyroxene characterize stage 4. Retrograde stages 5 and 6 consist of tremolite + Mg‐chlorite ± serpentine ± talc. A high‐temperature (～950 °C), spinel‐bearing protolith (stage 0), is identified on the basis of orthopyroxene + clinopyroxene + olivine + spinel + amphibole inclusions within stage 1 garnet. The P–T estimates for stage 1 are indicative of UHP conditions (3.2–3.3 GPa and 764–820 °C), whereas stage 2 is constrained between 726–788 °C and 2.6–2.9 GPa. Stage 3 records a decompression up to 1.1–1.3 GPa at 705–776 °C. Stages 4, 5 and 6 reflect uplift and cooling, the final estimates yielding values below 0.5 GPa at 300–400 °C. The retrograde P–T path is nearly isothermal from UHP conditions up to deep crustal levels, and becomes a cooling–unloading path from intermediate to shallow levels. The garnet‐bearing ultramafic rocks originated in the mantle wedge and were probably incorporated into the subduction zone with felsic and mafic rocks with which they shared the subsequent metamorphic and geodynamic evolution. The density and rheology of the subducted rocks are compatible with detachment of slices along the subduction channel and gravity‐driven exhumation.     

PGE and PGM in the Luanga mafic-ultramafic intrusion in Serra dos Carajás (Pará State, Brazil)

The late Archean, Luanga mafic-ultramafic complex intrudes an Archean greenstone belt, that is mainly composed of ultramafic and mafic metavolcanics. The Luanga intrusion consists of dunite, peridotite, gabbro and norite; chromitite seams and layers are present in the ultramafic rocks.A metamorphic overprint transformed the primary paragenesis into a serpentine-talc-chlorite-tremolite and magnetite association. The magnetite is commonly altered to Fe-hydroxides. Unaltered chromite commonly displays atoll-like textures and a chemical composition typical of stratiform chromites (Cr₂O₃ below 45 wt%).Base-metal sulfides, base-metal alloys, platimum-group minerals and platinum group element bearing phases are present in the form of inclusions in the silicate assemblages and in or on the edges of chromite grains. The main minerals detected are pentlandite, pyrrhotite, millerite, chalcopyrite and mackinawite, Fe---Ni alloy, braggite, sperrylite and platinum group elements (PGE) bearing sulfo-arsenides. Braggite is associated with the chromite, whereas sperrylite lies on the edges of or is included in silicates. The PGE content of the massive and disseminated chromities is dominated by Pt (up to 8900 ppb) and the chondrite-normalized PGE profile shows a cuspidal shape with a Pt peak.The main hypothesis for the source of the PGE-rich magma, which fractionated the chromitite-bearing ultramafic magma, consists of a relatively primitive mantle that partially melted in the late Archean.     

Evolution of the Pan-African Wadi Haimur metamorphic sole, Eastern Desert, Egypt

By comparison with the general features of metamorphic soles (e.g. vertical and lateral extension, metamorphic grade and diagnostic mineral parageneses, deformation and dominant rock types), it is inferred that the amphibolites, metagabbros and hornblendites of the Wadi Um Ghalaga–Wadi Haimur area in the southern part of the Eastern Desert of Egypt represent the metamorphic sole of the Wadi Haimur ophiolite belt. The overlying ultramafic rocks represent overthrusted mantle peridotite. Mineral compositions and thermobarometric studies indicate that the rocks of the metamorphic sole record metamorphic conditions typical of such an environment. The highest P – T conditions ( c . 700 °C and 6.5–8.5 kbar) are preserved in clinopyroxene amphibolites and garnet amphibolites from the top of the metamorphic sole, which is exposed in the southern part of the study area. The massive amphibolites and metagabbros further north (Wadi Haimur) represent the basal parts of the sole and show the lowest P – T  conditions (450–620 °C and 4.7–7.8 kbar). The sole is the product of dynamothermal metamorphism associated with the tectonic displacement of ultramafic rocks. Heat was derived mainly from the hot overlying mantle peridotites, and an inverted P – T  gradient was caused by dynamic shearing during ophiolite emplacement. Sm/Nd dating of whole-rock–metamorphic mineral pairs yields similar ages of c . 630 Ma for clinopyroxene and hornblende, which is interpreted as a lower age limit for ophiolite formation and an upper age limit for metamorphism. A younger Sm/Nd age for a garnet-bearing rock ( c . 590 Ma) is interpreted as reflecting a meaningful cooling age close to the metamorphic peak. Hornblende K/Ar ages in the range 570–550 Ma may reflect thermal events during late orogenic granite magmatism.     

Corundum-bearing amphibolites from the metamorphic basement of the Krivaja–Konjuh ultramafic massif (Dinaride Ophiolite Zone, Bosnia)

Summary ?Corundum-bearing amphibolites are part of the metamorphic basement of the Krivaja–Konjuh ultramafic massif in the Dinaride Ophiolite Zone in Bosnia. Pinkish corundum occurs as porphyroblasts and together with edenitic–pargasitic hornblende and anorthite within the amphibolites. Based on major and trace element contents, the protoliths of the corundum-bearing amphibolites were tholeiitic gabbro cumulates. Geothermobarometric estimations on the corundum-bearing amphibolites yielded preliminary P–T conditions of 620–830 °C and 6–10 kbar for amphibole and plagioclase inclusions inside corundum and 4.5–8 kbar for the main metamorphic assemblage. These estimates are thought to reflect the metamorphic conditions achieved during the Late Jurassic obduction of the Krivaja–Konjuh ultramafic massif onto ophiolite mélange. This study documents the mineralogy, petrology and geochemistry of these unusual corundum-bearing ophiolite-related edenite–pargasitic amphibolites. Received June 20, 2002; accepted October 22, 2002     

The nature and origin of Fe-Ti-P-rich rocks in the Qareaghaj mafic-ultramafic intrusion, NW Iran

Summary Fe-Ti-P-rich rocks (FTP) are unusual with respect to their mineralogy and bulk composition. Varieties of these rocks are mostly related to Proterozoic massif-type anorthosites and to a lesser extent to the upper parts of mafic-ultramafic intracratonic layered complexes and other igneous rock suites. We present results on the geology, mineralogy and geochemistry of a new occurrence of FTP, associated with mafic rocks in the northwestern part of Iran. The Qareaghaj mafic-ultramafic intrusion (QMUI) is a small igneous body situated between Palaeozoic sedimentary rocks and a Precambrian low grade metamorphic complex. The QMUI is composed mainly of non-mineralized mafic and apatite- and Fe-Ti oxide-rich ultramafic rocks. The mafic rocks, mainly coarse-grained gabbro, microgabbro and amphibolite, have a simple mineral assemblage (plagioclase + clinopyroxene + ilmenite) and based on field observations, mineralogy and chemical composition are comagmatic. The ultramafic rocks with high proportion of olivine (∼40–66 vol.%), apatite (∼0.1–16 vol.%), ilmenite (∼11–19 vol.%) and magnetite (∼2–13 vol.%), have unusual bulk compositions (e.g., SiO₂ ∼ 21–30 wt.%, total iron expressed as Fe₂O₃ ^tot ∼ 26–42 wt.%, TiO₂ ∼ 5–11 wt.%, MgO ∼ 9–20 wt.%, P₂O₅ up to 5.1 wt.%, Cr ∼ 40–160 ppm, Ni ∼ 7–73 ppm). The FTP forms numerous sill-like layers, ranging in thickness from ∼5 cm to few meters. These rocks, totally enclosed in mafic rocks with sharp and concordant contacts, show a magmatic lamination and follow the general NW–SE trend of QMUI. The apatite-rich ultramafic rocks makes up 90–95% of the total ultramafic outcrops and contain Mg-poor olivine (Mg# ∼ 40–58) and low-Mg spinel (Mg# ∼ 30–44) in contrast to apatite-poor ones (∼60–63 and ∼43–46, respectively). Field relationships, mineral compositions and geochemical data suggested that the FTP are not related to the mafic host rocks. On the contrary, they intruded latter into the gabbros during plastic, high temperature deformation in local shear zones. Fractional crystallization of P-rich ferrobasaltic parental magma at depth, probably in an open magmatic system, not far from the QMUI magma chamber, is considered as responsible for the formation of the evolved FTP in QMUI.     

P-T evolution of metapelites from the Bajgan complex in the Makran accretionary prism,south eastern Iran

The Bajgan Complex, one of the basement constituents of the arc massif in Iranian Makran forms a rugged, deeply incised terrain. The complex consists of pelitic schists with minor psammitic and basic schists, calc silicate rocks, amphibolites, marbles, metavolcanosediments, mafic and felsic intrusives as well as ultramafic rocks. Metapelitic rocks show an amphibolite facies regional metamorphism and contain garnet, biotite, white mica, quartz, albite ± rutile ± apatite. Thermobarometry of garnet schist yields pressure of more than 9 kbar and temperatures between 560 and 675 °C. The geothermal gradient obtained for the peak of regional metamorphism is 19 °C/km, corresponding to a depth of ca. 31 km. Replacement of garnet by chlorite and epidote suggest greenschist facies metamorphism due to a decrease in temperature and pressure through exhumation and retrograde metamorphism (370–450 °C and 3–6 kbar). The metapelitic rocks followed a ‘clockwise’ P–T path during metamorphism, consistent with thermal decline following tectonic thickening. The formation of medium-pressure metamorphic rocks is related to presence of active subduction of the Neotethys Oceanic lithosphere beneath Eurasia in the Makran.     

Constraints on retrograde metamorphism of UHP eclogites in North Qinling,Central China,from 40Ar/39Ar dating of amphibole and phengite

Coesite- and microdiamond- bearing ultra-high pressure (UHP) eclogites in the North Qinling terrane have been widely retrogressed to amphibolites. Previous geochronological studies on these UHP rocks mainly focused on the timing of peak eclogite facies metamorphism. The Kanfenggou UHP metamorphic domain is one of the best-preserved coesite-bearing eclogite occurrences in the North Qinling terrane. In this study, mafic amphibolites and host schists from this domain were collected for ⁴⁰Ar/³⁹Ar dating to constrain their retrograde evolution. Two generations of amphibole are recognized based on their mineral parageneses and ⁴⁰Ar/³⁹Ar ages. A first generation of amphibole from garnet amphibolites yielded irregularly-shaped age spectra with anomalously old apparent ages. Isochron ages of 484–473 Ma and initial ⁴⁰Ar/³⁶Ar ratios of 3695–774 are obtained from this generation of amphibole, indicating incorporation of excess argon. Second generation amphibole occurs in epidote amphibolites yielded flat age spectra with plateau ages of 464–462 Ma without evidence for excess argon. These ages suggest that the amphibolite-facies metamorphism has taken place as early as 484 Ma and lasted until 462 Ma for the North Qinling UHP metamorphic rocks. Phengite from the country-rock schists yielded ⁴⁰Ar/³⁹Ar plateau ages of 426–396 Ma, with higher phengite Si contents associated with the older the plateau ages. Based on our new ⁴⁰Ar/³⁹Ar ages and previous zircon UPb geochronological data, we construct a new detailed pressure-temperature-time (P-T-t) path illustrating the retrograde metamorphism and exhumation rate of the North Qinling eclogites and host schists. The P-T-t path suggests that these UHP metamorphic rocks experienced initial medium-to-high exhumation rates (ca. 8.7 mm/yr) during the Early Ordovician (489–484 Ma), which was mainly derived from buoyancy forces. Subsequently, the exhumation rate decreased gradually from ~0.8 to 0.3 mm/yr from 484 to 426 Ma, which was probably governed by extension and/or erosion.     

Petrogenesis of sapphirine-bearing metatroctolites from the Buck Creek ultramafic body,southern Appalachians

The Buck Creek ultramafic body, North Carolina, includes aluminous lenses that have been described as troctolites. These lenses preserve mineral assemblages which record several different stages of metamorphism. The first stage is characterized by anhydrous reactions between olivine and plagioclase to produce coronas of orthopyroxene+ clinopyroxene/spinel symplectite. Thermo barometric results indicate minimum pressures of c. 6 kbar and c. 800 ^oC. Sapphirine replaces spinel in some clinopyroxene symplectites, and occurs as anhedral grains within amphibole, observations which in combination suggest peak metamorphic conditions of c. 9-10 kbar and c. 850 ^oC. Sapphirine-bearing hydrous assemblages formed at the expense of the coronas, indicating a second metamorphic episode involving deeper burial, deformation and hydration. Schistose rocks from the margins of the lenses are composed of anorthite+amphibole+margarite+corundum, and probably record a later, lower P-T event. Whole rock analyses for the Buck Creek lenses suggest an accumulate protolith of magnesian olivine and calcic plagioclase. Trace element data for the troctolites are consistent with data for adjacent amphibolites in suggesting that the Buck Creek mafic and ultramafic cumulates crystallized from magmas derived from a mantle source similar to that which produces modern intraplate or rift-related basalts. We propose that the Buck Creek ultramafics represent basal cumulates(± uppermost mantle) from ocean crust formed in a marginal basin in the latest Precambrian. Subduction-induced burial to at least 18 km under dry conditions induced corona formation. Collisional events of the Taconic orogeny thrust the Buck Creek rocks into the orogenic pile to at least 30 km depth and hydrated them along zones of weakness, locally producing P-T -P_H2O conditions appropriate for formation of sapphirine and hydrated assemblages, but still preserving some dry symplectites.     

The mafic and ultramafic rocks of the Mocksville complex of Central North Carolina: Petrogenetic and tectonic implications

Located in the northwestern part of the Charlotte terrane of the Carolina zone in central North Carolina, the Mocksville complex is a tabular body which trends NE-SW and covers an area of approximately 500?km². It consists of late Proterozoic to early Paleozoic, moderately metamorphosed and variably deformed, mainly plutonic ultramafic, mafic and felsic rocks. The ultramafic rocks are pyroxenites, wehrlites, and hornblendites; the mafic rocks are metagabbros and amphibolites; and, the felsic rocks are granites and diorites. Field, geochemical, and geothermobarometry studies suggest that the igneous and metaigneous rocks of the Mocksville complex are likely to be genetically related, formed by calc-alkaline differentiation of mafic magma, and originated in a moderate pressure environment (~8?kbar). Based mainly on the study of volcanic rocks, the terranes of the Carolina zone have been interpreted as magmatic arc terranes in most tectonic models concerning the evolution of the southern Appalachian orogen. The geochemical features of the mafic and ultramafic plutonic rocks of the Mocksville complex corroborate the arc origin of the Charlotte terrane.     

The Kamuikotan zone in Hokkaido, Japan: tectonic mixing of high-pressure and low-pressure metamorphic rocks

Abstract. In the Kamuikotan zone, central Hokkaido, Japan, two distinct types of metamorphic rocks are tectonically mixed up, along with a great quantity of ultramafic rocks; one type consists of high-pressure metamorphic rocks, and the other of low-pressure ones. The high-pressure metamorphic rocks are divided into two categories. (1) Prograde greenschist to glaucophaneschist facies rocks derived from mudstone, sandstone, limestone, a variety of basic rocks such as pillow and massive lavas, hyaloclastite and tuff, and radiolarian (Valanginian to Hauterivian) chert, among which the basic rocks and the chert, and occasionally the sandstone, occur as incoherent blocks (or inclusions) enveloped by mudstone. (2) Retrograde amphibolites with minor metachert and glaucophane-calcite rock, which are tectonic (or exotic) blocks enclosed within prograde mudstone or serpentinite, or separated from these prograde rocks by faults. The K-Ar ages of the prograde metamorphic rocks (72, 107 and 116 Ma on phengitic muscovites) are younger than those of the retrograde rocks (109, 132, 135 and 145 Ma on muscovites, and 120 Ma on hornblende). The low-pressure metamorphic rocks consist of the mafic members of an ophiolite sequence with a capping of radiolarian (Tithonian) chert with the metamorphic grade ranging from the zeolite facies, through the greenschist (partly, actinolite-calcic plagioclase) facies to the amphibolite (partly, hornblende-granulite) facies. The low-pressure metamorphism has a number of similarities with that described for'ocean-floor'metamorphism. The tectonic evolution of such a mixed-up zone is discussed in relation to Mesozoic plate motion.     

Implication of corona formation in a metatroctolite to the granulite facies overprint of HP–UHP rocks in the Moldanubian Zone (Bohemian Massif)

Corona and inclusion textures of a metatroctolite at the contact between felsic granulite and migmatites of the Gföhl Unit from the Moldanubian Zone provide evidence of the magmatic and metamorphic evolution of the rocks. Numerous diopside inclusions (1–10 μm, maximum 20 μm in size) in plagioclase of anorthite composition represent primary magmatic textures. Triple junctions between the plagioclase grains in the matrix are occupied by amphibole, probably pseudomorphs after clinopyroxene. The coronae consist of a core of orthopyroxene, with two or three zones (layers); the innermost is characterized by calcic amphibole with minor spinel and relicts of clinopyroxene, the next zone consists of symplectite of amphibole with spinel, sapphirine and accessory corundum, and the outermost is formed by garnet and amphibole with relicts of spinel. The orthopyroxene forms a monomineralic aggregate that may contain a cluster of serpentine in the core, suggesting its formation after olivine. Based on mineral textures and thermobarometric calculations, the troctolite crystallized in the middle to lower crust and the coronae were formed during three different metamorphic stages. The first stage relates to a subsolidus reaction between olivine and anorthite to form orthopyroxene. The second stage involving amphibole formation suggests the presence of a fluid that resulted in the replacement of igneous orthopyroxene and governed the reaction orthopyroxene + anorthite = amphibole + spinel. The last stage of corona formation with amphibole + spinel + sapphirine indicates granulite facies conditions. Garnet enclosing spinel, and its occurrence along the rim of the coronae in contact with anorthite, suggests that its formation occurred either during cooling or both cooling and compression but still at granulite facies conditions. The zircon U–Pb data indicate Variscan ages for both the troctolite crystallization (c. 360 Ma) and corona formation during granulite facies metamorphism (c. 340 Ma) in the Gföhl Unit. The intrusion of troctolite and other Variscan mafic and ultramafic rocks is interpreted as a potential heat source for amphibolite–granulite facies metamorphism that led to partial re‐equilibration of earlier high‐ to ultrahigh‐P metamorphic rocks in the Moldanubian Zone. These petrological and geochronological data constrain the formation of HP–UHP rocks and arc‐related plutonic complex to westward subduction of the Moldanubian plate during the Variscan orogeny. After exhumation to lower and/or middle crust, the HP–UHP rocks underwent heating due to intrusion of mafic and ultramafic magma that was generated by slab breakoff and mantle upwelling.     

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.     

Mafic and ultramafic magmatism and associated mineralization in the Dharwar craton,southern India

Evidence of mafic and ultramafic magmatism exists in many parts of the Dharwar craton which is divided into two blocks, the West Dharwar Craton (WDC) and the East Dharwar Craton (EDC). The mafic-ultramafic rocks occur in supracrustal/greenstone belts and in numerous enclaves and slivers in the WDC. The oldest recorded maficultramafic rocks, which are mainly komatiitic in nature, are preserved in the Sargur Group which is more than 3.3–3.4 Ga old, the youngest being manifested by 63–76 Ma old mafic dyke magmatism, possibly related to Deccan volcanism. In the Sargur Group, ultramafics rocks greatly dominate over mafic lithological units. Both extrusive and intrusive varieties, the latter in the form of differentiated layered complexes, occur. Mafic volcanics exists in all the greenstone belts of the eastern block and in the Bababudan and Western Ghats belts of the western block. In addition to the Sargur Group where stratigraphic sequences are unclear, mafic magmatism is recorded in three different formations of the Bababudan Group and two sub-divisions of the Shimoga and Chitradurga Groups where basaltic flows are conspicuous. In the well studied greenstone belts of Kolar and Hutti in the EDC, three to four different Formations of mafic volcanic rocks have been mapped. Isotopic dating has indicated that while mafic magmatism in the greenstone belts of the EDC covers only a short time span of between 2.65 to 2.75 Ga, those in the Dharwar Supergroup of the WDC cover a much longer time span from 3.35 to 2.5 Ga. Mafic dyke magmatism has taken place repeatedly from 2.45 Ga to about 1.0 Ga, but, the peak of emplacement was between 1.8 and 1.4 Ga when the densely developed swarms on the western and south western portions of the Cuddapah Basin and in the central part of Karnataka, were intruded. Emplacement of potassic ultramafic magma in the form of kimberlite-lamproite which is confined to the EDC, is a later magmatic event that took place between 1.4 Ga and 0.8 Ga. From a mineralization perspective, mafic magmatism of the supracrustal groups of the WDC and the greenstone belts of the EDC are the most important. V-Ti-magnetite bands constitute the most common deposit type recorded in the mafic-ultramafic complexes of the Sargur Group with commercially exploitable chromite deposits occurring in a number of belts. PGE mineralization of possible commercial value has so far been recorded in a single mafic-ultramafic complex, while copper-nickel mineralization occurs at certain localities in the Sargur and Chitradurga Groups. Gold mineralization hosted by mafic (occasionally ultramafic) rocks has been noted in many of the old workings located in supracrustal groups of rocks in the WDC and in the greenstone belts of EDC. Economically exploitable mineralization, however, occurs mainly in the greenstone belts of the Kolar, Ramagiri-Penkacherla and Hutti-Maski and along the eastern margin of the Chitradurga belt, where it is associated with a major N-S striking thrust zone separating the WDC from the EDC. Gold deposits of the eastern greenstone belts are comparable to those of the younger greenstone belts of Canada, Zimbabwe and Australia where the mineralization is associated with quartz carbonate veins often in iron-rich metabasic rocks. The gold was emplaced as hydrothermal fluids, derived from early komatiitic and tholeiitic magmas, and injected into suitable dilatent structures. The other common type of mineralization associated with the ultramafic rocks of the Sargur Group and supracrustal belts, particularly of the WDC, are asbestos and soapstone, related to autometamorphism/metasomatism. Ruby/sapphire deposits occur in places at the contacts of ultramafic rocks with the Peninsular Gneiss, and are related to contact metamorphism and metasomatism. Mineable magnesite deposits related to low-temperature hydrothermal/lateritic alteration exist in the zone of weathering, particularly in the more olivine-rich rocks. Recent spurt in diamond exploration is offering promise of discovering economically workable diamondiferous kimberlite/lamproite intrusions in the EDC.     

华北克拉通新太古代板块俯冲作用:来自山西云中山地区变质高镁火成岩组合的证据

在华北克拉通中部的山西云中山地区,新太古代花岗闪长质片麻岩中存在一些超镁铁质岩-镁铁质岩块及由斜长角闪岩、角闪变粒岩、石英岩和石榴夕线黑云片岩等岩石类型构成的变质表壳岩残片,其中的超镁铁质-镁铁质岩、斜长角闪岩和角闪变粒岩构成一套高镁火成岩组合。超镁铁质岩已变质为橄榄绿泥阳起片岩等岩石类型,呈变余斑状结构,橄榄石斑晶仍有保存;岩石SiO_2含量为39.22%～44.99%,Al_2O_3为8.82%～13.47%,Mg O为19.24%～22.13%,Na_2O+K_2O=0.71%～1.11%,CaO为5.75%～8.42%;Al_2O_3/TiO_2=14.8～17.4,CaO/Al_2O_3=0.60～0.84;化学成分上与科马提岩有一定的相似性。与之紧密伴生的斜长角闪岩也具有高镁特征,Mg O含量为11.28%～15.09%,铝、硅和碱质均偏低,具正铕异常,显示堆晶辉长岩的特征。非高镁斜长角闪岩有相对高的铝、硅和碱质,其原岩应为钙碱性玄武岩。角闪变粒岩样品的SiO_2含量为54.21%～55.71%,Al_2O_3为14.24%～15.49%,Mg O为6.26%～8.28%,Fe OT/Mg O=1.11～1.58,高钠低钾,Na_2O+K_2O=3.7%～4.78%,Na_2O/K_2O=5.15%～13.13,Mg#=53.0～61.5,属于高镁安山岩。由超镁铁质质岩-斜长角闪岩-角闪变粒岩构成的变质高镁火山岩组合具有钙碱性系列趋势。超镁铁质岩稀土元素含量总量较低,具有轻稀土富集和重稀土亏损的稀土型式;斜长角闪岩与超镁铁质岩比较,除富集大离子亲石元素和Cr、Ni明显较低外,具有相似的微量元素图谱形态。三种岩石类型在微量元素蛛网图上均显示出Ta、Nb、Ti负异常和Pb正异常。野外产状和岩石地球化学特征表明超镁铁质岩和高镁斜长角闪岩属于阿拉斯加型杂岩体,角闪变粒岩属于赞岐岩质高镁安山岩。在Zr/Nb-Nb/Th和Nb/Y-Zr/Y构造环境判别图解上显示出与俯冲相关的演化趋势,在Hf-Th-Ta、Nb/La-(La/Sm)N和Th/Yb-Nb/Yb图解上也落在岛弧钙碱性岩石区域。以上特征表明高镁火成岩组合形成于与板块俯冲相关的岛弧构造背景。野外地质关系和锆石U-Pb年龄限定高镁火成岩组合形成时代在～2.5Ga。云中山地区阿拉斯加型镁铁质-超镁铁质杂岩与赞岐岩质高镁安山岩共生,表明该地区存在新太古代的板块俯冲作用,为太古宙存在板块构造机制提供了新证据。