西藏冈底斯带东段石炭纪构造环境讨论

冈底斯带东段石炭纪构造环境存有争议。本文通过岩石地球化学分析,发现石炭纪诺错组和来姑组火山岩具有碱性系列向拉斑系列过渡的特征,具有双峰火山岩特点;石炭纪玄武岩样品的平均化学成分与大陆拉斑玄武岩的平均值较为接近;微量元素除Ti、Yb、Y之外,其他元素都富集,其中Rb、Th、Ce富集程度相对较高;稀土元素特征表现为轻稀土富集,铕、铈异常不明显。显示出石炭纪玄武岩的地球化学特征均与大陆拉斑玄武岩相似。构造环境判别图解显示,石炭纪玄武岩样品主要落入板内环境的大陆拉张带(或初始裂谷)玄武岩区。因而认为冈底斯东段在石炭纪属于陆内裂谷或被动陆缘裂谷环境。     

Petrogenesis of early Silurian intrusions in the Sanchakou area of Eastern Tianshan,Northwest China,and tectonic implications: geochronological,geochemical, and Hf isotopic evidence

ABSTRACT

Palaeozoic intrusions in Eastern Tianshan are important for understanding the evolution of the Central Asian Orogenic Belt (CAOB). The Sanchakou intrusions situated in Eastern Tianshan (southern CAOB), are mainly quartz diorite and granodiorite. A comprehensive study of zircon U–Pb ages, zircon trace elements, whole-rock geochemistry, and Lu–Hf isotopes were carried out for the Sanchakou intrusive rocks. LA-ICP-MS zircon U–Pb dating yielded crystallization ages of 439.7 ± 2.5 Ma (MSWD = 0.63, n = 21) for the quartz diorite, and 430.9 ± 2.5 Ma (MSWD = 0.21, n = 21) and 425.5 ± 2.7 Ma (MSWD = 0.04; n = 20) for the granodiorites. These data, in combination with other Silurian ages reported for the intrusive suites from Eastern Tianshan, indicate an early Palaeozoic magmatic event in the orogen. In situ zircon Hf isotope data for the Sanchakou quartz diorite shows ε_Hf(t) values of +11.2 to +19.6, and the two granodioritic samples exhibit similar ε_Hf(t) values from +13.0 to +19.5. The Sanchakou plutons show metaluminous to weakly peraluminous, arc-type geochemical and low-K tholeiite affinities, and display trace element patterns characterized by enrichment in K, Ba, Sr, and Sm, and depletion in Nb, Ta, Pb, and Ti. The geochemical and isotopic signatures indicate that the Sanchakou dioritic and granodioritic magmas were sourced from a subducted oceanic slab, and subsequently underwent some interaction with peridotite in the mantle wedge. Combined with the regional geological history, we suggest the Sanchakou intrusions formed due to the northward subduction of the Palaeo-Tianshan Ocean beneath the Dananhu–Tousuquan arc during early Silurian time.     

Origin and geodynamic environments of the metamorphic sole rocks from the İzmir–Ankara–Erzincan suture zone (Tokat,northern Turkey)

ABSTRACT

The Late Cretaceous accretionary complex of the ?zmir–Ankara–Erzincan suture zone, near Artova, is composed mainly of peridotites (variably serpentinized), amphibolite, garnet-micaschist, calc-schist, marble, basalt, sandstones, neritic limestones. The metamorphic rocks were interpreted as the metamorphic sole rocks occurring at the base of mantle tectonites, because: (i) amphibolites were observed together with the serpentinized peridotites suggesting their occurrences in the oceanic environment; (ii) foliation in amphibolites and serpentinized peridotites run subparallel to each other; (iii) all these metamorphic rocks and serpentinized peridotites are cross-cut by the unmetamorphosed dolerite dikes with island arc tholeiite-like chemistry. Geochemical characteristics of the amphibolites display enriched mid-ocean ridge basalt (E-MORB)- and ocean island basalt (OIB)-like signatures. The dolerite dikes, on the other hand, yield an island arc tholeiite-like composition. Geothermobarometric investigations of the metamorphic sole rocks suggest that the metamorphic temperature was ~650 ± 30°C and the pressure condition was less than 0.5 GPa. Dating of hornblende grains from amphibolite yielded age values ranging from 139 ± 11 to 157 ± 3.6 Ma (2σ). The oldest weighted average age value is regarded as approximating the timing of the intra-oceanic subduction. These cooling ages were interpreted to be the intra-oceanic subduction/thrusting time of the ?zmir–Ankara–Erzincan oceanic domain.     

Review Section

ABSTRACT

The petrology, geochronology, and geochemistry of the early Permian volcanic rocks from Houtoumiao area, south Xiwuqi County in central Inner Mongolia of China, are studied to elucidate the early Permian tectonic setting of the region. The volcanic rocks, which are interbedded with sandstone, feature both mafic and felsic compositions and show a bimodal nature. Zircon U–Pb dating reveals that the volcanic rocks formed at 274–278 Ma, similar to the ages of bimodal magmatism in neighbouring areas. The mafic rocks are composed of tholeiitic basalt, basaltic andesite, basaltic trachyandesite, and trachyandesite. They are rich in Th, U, and LILEs, depleted in HFSEs Nb, Ta, and Ti, and have positive ε_Nd(t) values (+3.6 to +7.9). Geochemical analyses indicate that the mafic rocks originated from metasomatized lithospheric mantle. The felsic volcanic rocks are mainly rhyolite, with minor trachyte and dacite. They have different evolutionary tendencies of major elements, chondrite-normalized REE patterns, and isotopic compositions from the mafic volcanic rocks, which preclude formation by fractional crystallization of mafic melts. The ε_Nd(t) values of the felsic rocks are similar to those of the Carboniferous Baolidao arc rocks in the region. It is suggested that Permian felsic melts originated from the partial melting of Carboniferous juvenile arc-related rocks. By comparison with typical Cenozoic bimodal volcanism associated with several tectonic settings, including rift, post-collisional setting, back-arc basin, and the Basin and Range, USA, the bimodal volcanic rocks in central Inner Mongolia display similar petrological and geochemical characteristics to the rocks from back-arc basin and the Basin and Range, USA. Based on the analysis of regional geological data, it is inferred that the early Permian bimodal volcanic rocks in the study area formed on an extensional continental margin of the Siberian palaeoplate after late Carboniferous subduction–accretion.     

Geochemistry and detrital modes of Proterozoic sedimentary rocks,Bayana Basin,north Delhi fold belt: implications for provenance and source-area weathering

Bayana Basin, sited along the eastern margin of the north Delhi fold belt of the Aravalli Craton, contains an ～3000?m-thick sequence comprising one volcanic and seven sedimentary formations of the Delhi Supergroup. The sedimentary units are the Nithar, Jogipura, Badalgarh, Bayana, Damdama, Kushalgarh, and Weir formations in order of decreasing age. Petrographic study of the sandstones as well as major and trace elements (including rare earth elements) and bulk-rock analyses of the shales and sandstones allow the determination of their provenance, source-rock weathering, and basinal tectonic setting. The sandstones are quartz rich and were derived mainly from exhumed granitoids typical of a craton interior. Geochemical patterns of the sandstones and shales are similar. However, trace element abundances are low in sandstones, probably due to quartz dilution. The coarser clastic Damdama and Weir sandstones, which occur at higher stratigraphic levels, have strikingly low trace element concentrations compared with the underlying Bayana and Badalgarh sandstones. All samples show uniform LREE-enriched patterns with negative Eu-anomalies (Eu/Eu*?=?0.16–0.23) and are similar to those of post-Archaean Australian shales (PAAS). However, the (La/Yb) _n ratios (averages 11–18) of all the sedimentary units are higher than those of PAAS, except for the Bayana Sandstone, which has low values (average 6.77). The chemical index of alteration (70–78) and the plagioclase index of alteration (87–97) values and the A–CN–K diagram suggest moderate to intense weathering of the source area.

The provenance analyses indicate that basin sedimentation was discontinuous. It received input from a terrain comprising granitoids, mafic rocks, sedimentary sequences, and tonalite-trondhjemite-granodiorite (TTG) suites. The Nithar and Badalgarh sandstones received input from a source consisting predominantly of granitoids. The succeeding Damdama and Weir sandstones received debris from granitoids and TTG in different proportions. The Kushalgarh shale was possibly derived from a source consisting granites and mafic rocks with a TTG component. The pre-existing sedimentary formations also contributed intermittently during the different phases of sedimentation.

Bulk-rock geochemical data suggest Mesoarchaean gneisses and late Archaean granites of BGC/BGGC (Banded Gneissic Complex/Bundelkhand Granitic Gneiss Complex) basement as possible source terrains. These data indicate deposition in a continental rift setting. The coeval formation of many rift-related Proterozoic sedimentary basins in the BGC/BGGC terrain suggests that the North Indian Craton underwent major intracratonic extension during Proterozoic time, probably triggering the break up of Earth's first supercontinent.     

Petrochemistry of Mesozoic mafic intrusions related to the Paraná Magmatic Province,Uruguay

The Cuaró Formation is part of the sequence of Mesozoic mafic intrusions related to the Early Cretaceous break-up of Gondwana and represents the southernmost occurrences within the Paraná Magmatic Province in Uruguay. We present field data, petrography and lithogeochemical results regarding these dike swarms and sills that crop out in the southern extreme of the Paraná Basin. Dolerites and sills mainly exhibit glomeroporphyritic textures; the phenocrysts consist of plagioclase, clinopyroxene, relicts of olivine and titaniferous magnetite. Bulk-rock geochemical analyses allowed their classification as low-Ti subalkaline tholeiitic basalts and andesitic basalts. Trace element data indicated that the protoliths of these intrusions include the subcontinental lithospheric mantle, as is generally recognized for other Gondwana-related continental flood basalt provinces.     

Messinian palaeoclimate and palaeo-environment in the western Mediterranean realm: insights from the geochemistry of continental deposits of NW Sardinia (Italy)

To evaluate the palaeo-environmental parameters of a portion of the Sardinia–Corsica microplate during the Messinian drop in sea level, we examined the chemistry and mineralogy of upper Tortonian–lower Messinian (late Miocene) clayey continental deposits from NW Sardinia. Differences exist between the uppermost part of the succession, which is devoid of carbonate phases, and the lower part, reflecting changes in provenance and climate. The carbonate-free samples were probably derived from quartzite of the metamorphic basement and were deposited under a climate characterized by alternating dry and relatively wet periods. The other samples were derived from basement phyllite and were deposited under a warm, dry climate that promoted the capillary rise of Ca²⁺ and bicarbonate from a shallow water table, and therefore, the precipitation of carbonate. This part of the succession contains both calcite and dolomite. The presence of barite indicates an important concentration of SO₄ ^2? in the solution from which the CaMg(CO₃)₂ precipitated. The formation of dolomite under hypersaline conditions may be explained by bacterial degradation of organic matter, which produced CO₂ and ammonia, thereby increasing the solution alkalinity. The succession formed in an oxic environment, except for a calcite-rich level that formed under relatively reducing conditions. For this level, the large amount of calcite and the lack of dolomite indicate an alkaline environment and a very low Mg²⁺/Ca²⁺ ratio in the soil solution. These observations, coupled with the reducing conditions, indicate the availability of large amounts of degraded organic matter, probably related to a period typified by a wetter climate.     

Geochronology,geochemistry, Hf isotopic compositions and formation mechanism of radial mafic dikes in northern Tibet

A large mafic dike swarm is radially distributed in southern Qiangtang. Three typical samples were selected for geochronology, geochemistry, and Hf isotopic analysis. Zircon U–Pb dating indicates that the three dikes formed at 291 ± 2, 292 ± 3, and 300 ± 2 Ma. Whole-rock compositions show that the southern Qiangtang mafic dikes are alkaline, Fe + Ti rich, and exhibit relative enrichment in light rare-earth elements. The ratios of incompatible elements are similar to those of oceanic island and Emeishan basalts. Geochemical diagrams show that the dikes erupted in an intraplate environment. Zircon Hf isotopic data suggest that magma that produced the mafic dikes was derived from a depleted mantle source. The geochemical characteristics of the dikes approximate that of eruption products of a brief period of mantle plume activity (300–280 Ma). According to eight geologic maps of Qiangtang, the mafic dikes crop out over an area of 150 km from north to south and 500 km from east to west, radiating outward from Mayigangri. We conclude that the mafic dikes in southern Qiangtang are related to the combined effect of Permian plate motions and mantle plume activity, and the Mayigangri area overlies the hot spot. Furthermore, the mantle plume in southern Qiangtang may have propelled the closing of the Palaeo-Tethys Ocean.     

Origin of the Yinshan epithermal-porphyry Cu–Au–Pb–Zn–Ag deposit,southeastern China: insights from geochemistry,Sr–Nd and zircon U–Pb–Hf–O isotopes

The Yinshan deposit is a large epithermal-porphyry polymetallic deposit, and the timing and petrogenesis of ore-hosting porphyries have been hotly debated. We present new results from geochemical, whole-rock Sr–Nd and zircon U–Pb–Hf–O isotopic investigations. Zircon U–Pb data demonstrate that the quartz porphyry, dacitic porphyry, and quartz dioritic porphyry formed at ?172.2 ± 0.4 Ma, ?171.7 ± 0.5 Ma, and ?170.9 ± 0.3 Ma, respectively. Inherited zircon cores show significant age spreads from ?730 to ?1390 Ma. Geochemically, they are high-K calc-alkaline or shoshonitic rocks with arc-like trace element patterns. They have similar whole-rock Nd and zircon Hf isotopic compositions, yet an increasing trend in ?_Nd(t) and ?_Hf(t) values typifies the suite. Older (inherited) zircons of the three porphyries display Hf compositions comparable to those of the Jiangnan Orogen basement rocks. In situ zircon oxygen isotopic analyses reveal that they have similar oxygen isotopic compositions, which are close to those of mantle zircons. Moreover, a decreasing trend of δ¹⁸O values is present. We propose that the ore-related porphyries of the Yinshan deposit were emplaced contemporaneously and derived from partial melting of Neoproterozoic arc-derived mafic (or ultra-mafic) rocks. Modelling suggests that the quartz porphyries, dacitic porphyries, and quartz dioritic porphyries experienced ?25%, ?10%, and ?10% crustal contaminations by Shuangqiaoshan rocks. Our study provides important constraints on mantle–crust interaction in the genesis of polymetallic mineralization associated with Mesozoic magmatism in southeastern China.     

Age of metamorphic rock masses in Laba Basin,North Caucasus

A Neoarchaean sanukitoid pluton that was intruded into the base of the Guyang greenstone belt in the Yinshan Block of the North China Craton hosts a number of hornblendite enclaves. Geochemically, the pluton is characterized by high MgO, Mg#, Cr, Ni, large-ion lithophile element (LILE), and heavy rare earth element (HREE) contents and low TiO₂ and high-field strength element (HFSE) contents, and has relatively low Sr/Y ratios and negative Eu anomalies. Whole-rock Sr–Nd isotopic analyses indicate that it has ε_Nd(t) values of +1.4 to +2.0. These geochemical and isotopic characteristic suggest that the sanukitoid was formed under a low-pressure and high-temperature environment by slab melting and assimilation of hornblendite enclaves. The hornblendite enclaves show high MgO, Mg#, Cr, and Ni contents, high and variable K₂O, LREE, and Th contents, enrichment in LILEs and LREEs, and depletion in HFSEs. They have high Y contents and relatively low Sr/Y values, strongly negative Eu anomalies, and whole-rock ε_Nd(t) values of +1.0 to +1.9. The geochemical and isotopic characteristics indicate that these enclaves might have been derived from a mixed source of an enriched mantle, metasomatized by melts expelled from subducted sediments in a high-temperature, low-pressure environment. To explain these characteristics, a ridge subduction model is proposed for the formation of the sanukitoid and hornblendite in the Yinshan Block in the Neoarchaean.