哈尔里克山晚古生代混合岩地质特征、年代学及其构造意义

哈尔里克山位于天山造山带东北缘,是古亚洲洋板片俯冲、弧—陆(或弧—弧)增生拼贴造山作用的产物.出露于哈尔里克山南麓的中—高级变质带中发育有混合岩,其成因和时代尚无详细研究.文章对哈尔里克变质带中的混合岩进行了野外岩相—构造分析与LA-ICP-MS锆石U-Pb年代学研究.结果显示,该混合岩与高级变质沉积岩紧密伴生,可能是...     

The origin of oceanic plagiogranites from the karmoy ophiolite,western Norway

Both field relationships and geochemical characteristics indicate two suites of plagiogranitic and related rocks coexisting in the higher parts of the Karmoy ophiolite of western Norway. The plutonic zone of this ophiolite can be subdivided into three complexes; the East-Karmoy Igneous Complex, the Visnes High Level Complex and the Veavagen Igneous Complex and plagiogranitic rocks are well developed in the first two of these.Within the East-Karmoy Igneous Complex, plagiogranites are associated with high temperature, pre-basic dyke, shear zones. Rare earth element modelling indicates that these plagiogranites were derived by anatexis of amphibolite (hydrated diabase) assuming a starting material consisting of 40% hornblende and 60% plagioclase and that batch melting occurred within the stability field of hornblende.In comparison, plagiogranite occurs in a number of bodies in the upper part of the Visnes High Level Complex and forms a sandwich horizon together with biotite diorites and epidosites between a roof assemblage of dykes, microgabbros and magnetite gabbros, and a floor assemblage of layered and non-layered gabbros. The R.E.E. modelling of the petrogenesis of this series of plagiogranites indicates that they were derived by filter pressing of a differentiated interstitial liquid to the vari-textured gabbros, although the distribution of highly hygromagmatophile elements such as K, Rb, Ba, etc. cannot be explained satisfactorily by this model alone. Depletion in these elements appears to be an autometasomatic effect.     

Paleozoic collisional and intraplate granitoids of the Baikal area: comparative geochemistry and petrogenesis

Early Paleozoic granitoids of autochthonous and allochthonous facies in the Baikal area (Ol’khon Island, Khamar-Daban Ridge) are in close spatial association with gneisses, migmatites, and plagiogranites and are usually confined to granite–gneiss domes. They are virtually not subjected to magmatic differentiation. Formation of granitoids of the Solzan massif and Sharanur complex lasted 26–28 Myr, which might be considered an indicator of collisional granitoid magmatism. Collisional granitoids of different provinces have a series of indicative features: They are peraluminous and highly potassic and are enriched in crustal elements (Rb, Pb, and Th) but sometimes have low contents of volatiles. In contrast to collisional magmatism, petrogenesis of intraplate granitoids does not depend on the composition and age of the enclosing rocks. The geochemical evolution of intraplate granitoid magmatism in the Baikal area is expressed as an increase in contents of F, Li, Rb, Cs, Sn, Be, Ta, Zr, and Pb and a decrease in contents of Ba, Sr, Zn, Th, and U during the differentiation of multiphase intrusions. The geochemical diversity of these granitoids formed both from crustal and from mantle sources and as a result of the mantle–crust interaction, might be due to the effect of plume on the geologic evolution of intraplate magmatism. The wide range of compositions and geochemical types of igneous rocks (from alkali and subalkalic to rare-metal granitoids) within the Late Paleozoic Baikal magmatism area suggests its high ore potential.     

Alkali metasomatism as a process for trondhjemite genesis: evidence from Aspromonte Unit, north-eastern Peloritani, Sicily

Summary Rocks of trondhjemitic composition are widespread in the North-Eastern Peloritani Belt within the Aspromonte Unit, a Hercynian medium- to high-grade metamorphic complex intruded by late-Hercynian peraluminous granites and later affected by MP/LT Alpine metamorphism. Among these trondhjemitic bodies, the Pizzo Bottino trondhjemites form one of the largest, outcropping over about 6km² and up to 400m thick. These rocks display concordant to discordant relationships with associated metamorphic rocks and are often cut by late-Hercynian leucogranitic dykes. The field, petrographic and geochemical features of these trondhjemites are consistent with an igneous origin. Petrographic and geochemical evidences suggest that the trondhjemitic character of the Pizzo Bottino rocks is due to an alkali metasomatism process involving cationic exchange of Na and Ca for K and consequent replacement of K-feldspar by oligoclase in the original granitoids. The major and trace element contents of the Pizzo Bottino trondhjemites are in fact comparable to those of the peraluminous late-Hercynian granitoids from the southern Calabrian-Peloritani Arc (CPA), when the elements directly involved in the alkali metasomatism process (Na, Ca, K, Sr, Ba, Rb) are not considered. The behaviour of REE elements, plus Th and U, also seems to be partially controlled by metasomatic processes, because their abundances vary with the K/Na ratio. Metasomatism seems to be the only viable mechanism involved in the genesis of the Pizzo Bottino trondhjemites. Other trondhjemite generation processes such as fractionation from basaltic parents and partial melting of metabasaltic or metasedimentary sources are ruled out on geological, petrographic and isotopic (Sr, Nd) grounds. Lastly, regional considerations place the metasomatic event during the late Hercynian, after the emplacement of the original granitoids and preceding the intrusion of the leucogranitic dykes, which are not affected by metasomatism.     

Petrology of basic and intermediate orogenic granitoids from the Sila Massif (Calabria,southern Italy)

Hercynian gabbroic, dioritic and tonalitic rocks crop out in the neighbourhood of Rovale (Sila Grande, Calabria). They make up a crude rectangular outcrop with the western part consisting of gabbroic rocks and the eastern of dioritic and tonalitic rocks. They come into contact with medium to high grade metapelites on the western side and with heterogeneous granodiorites on the other sides. In the gabbroic body both opx ± ol bearing cumulates and amphibole differentiates occur and are characterized by the widespread presence of brown pargasite. Sporadic magmatic to subsolidus corona textures between olivine and plagioclase or orthopyroxene and plagioclase can be observed and their preservation clearly suggests a post-tectonic emplacement for the gabbroic magma. Diorites and tonalites display hypidiomorphic textures free of olivine and orthopyroxene and bearing green Mg-hornblende. The granitoids, on the basis of chemical data, display orogenic features of the continent-continent collision type. The gabbroic rocks have high Al tholeiitic composition and fractionation of orthopyroxene and plagioclase played an important part in their evolution. The Rb/Sr isochron method did not give a precise emplacement age for the granitoids as a whole. Initial ⁸⁷Sr/⁸⁶Sr ratios (at 290 Ma) are higher in the gabbroic body (0.7091–0.7095) than in diorites and tonalites (0.7083–0.7092). Thus gabbroic rocks appear more displaced than diorites and tonalites towards crustal isotopic composition. The eNd data seem to confirm this feature, thus suggesting that the gabbroic rocks and diorites derived from distinct mantle magma batches. Interestingly, small isotropic gabbroic masses occur within the diorites and show general features that allow them to be considered as possibly parental with respect to the host diorites. The evolution to the dioritic composition might have occurred through fractionation and minor mixing with a more acidic component such as the northern granodiorites. Geochemical, Sr and Nd isotopic data indicate a scenario of a composite plutonic body formed by distinct magma batches of mixed crust and mantle origin.     

Geochemistry of the Chakradharpur granite—Gneiss complex—A Precambrian trondhjemite body from West Singhbhum,Eastern India

The Chakradharpur Granite—Gneiss complex (CKPG) is exposed as an elliptical body within the arcuate metamorphic belt sandwiched between the Singhbhum Granite in the south and the Chotonagpur Granite—Gneiss to the north. It consists of an older bimodal suite of grey gneiss and amphibolites, intruded by a younger unit of pegmatitic granite. The bimodal suite represents the basement to the enveloping metasediments.The average major-element chemistry of the grey gneiss conforms to the definition of trondhjemite and includes both low-Al₂O₃ and high-Al₂O₃ types. The amphibolites can be grouped into a low-MgO and a high-MgO type. Rocks of the younger unit range in composition from granodiorite to quartz monzonite. The two granitic units also differ significantly in their Rb, Sr and Ba contents, and in the REE distribution pattern. The grey gneiss shows a highly fractionated REE pattern and a distinct positive Eu anomaly, with Eu/Eu^* values increasing with increase in SiO₂ %. In samples of the younger granite, the REE pattern is less fractionated, with higher HREE abundance relative to the grey gneiss and usually shows a negative Eu anomaly. The two types of REE patterns in amphibolites are interpreted to represent the two broad groups identified on the basis of major element chemistry.On the basis of chemical data, a two-stage partial melting model for the genesis of grey gneiss is suggested, involving separation of hornblende and varying amounts of plagioclase in the residual phase. Varying amounts of plagioclase in the residuum result in the wide range of Al₂O₃ content of the partial melt from which the trondhjemites crystallised. Residual hornblende produces the highly fractionated REE pattern, but a relatively higher HREE content of the trondhjemites compared to those produced by separation of garnet in the residual phase. The high level of Ba together with moderate levels of Sr in the trondhjemites can also be explained by plagioclase in the residue, whose effectiveness in partitioning Ba compared to Sr is lower. Of the two groups of amphibolites, the low-MgO type shows relative depletion of LREE compared to the high-MgO type. It contains varying amounts of plagioclase and is tentatively suggested to represent the residue. The other group, with a slightly fractionated REE pattern (Ce_N/ Yb_N = 2.01), is generally considered to represent the source material for the trondhjemites. This may have been generated by 5–15% partial melting of mantle peridotites, containing higher concentrations of LIL elements than those which produced average Precambrian tholeiites. This first phase of partial melting resulted in the slightly fractionated REE pattern of these amphibolites. Derivation of the younger granitic unit from the trondhjemites can be ruled out on the basis of REE data alone. REE data suggest partial melting of metasediments to be the origin of these rocks. It is also possible that deeply buried volcanic rocks, similar to calc-alkaline components of greenstone belts, are the parent for this component.     

完达山造山带蛇绿混杂岩中变质基性岩的地球化学特征及其地质意义

完达山造山带蛇绿混杂岩中变质基性岩原岩主要有两种类型:在大顶子山和仙人台附近蛇绿岩带中的变质玄武岩地球化学具有T iO2含量偏高,高场强元素N b,T a,Z r,H f等富集,大离子亲石元素R b,Sr,B a等相对富集;LREE富集,体现出洋岛型玄武岩的特征。在靠近跃进山断裂带的跃进山变质杂岩带中采集的变质基性岩原岩具有洋中脊玄武岩的地球化学特征,但R b,B a,Th相对富集,R b,B a,Th都是活跃于地壳中的元素,这些元素丰度值高,与板块拼贴岩石受到明显的蚀变作用有关。     

Petrology of plagiogranites of the Yenisei Batholith, Western Sayan

The tonalite-plagiogranite (tonalite-trondhjemite) association only occasionally occurs in the form of large granitoid bodies, such as the Yenisei Batholith (>500 km² in area). The granitoids of the Yenisei Batholith belong to Na-rich tholeiitic rock series and differ from granitoids of the calc-alkaline series in having lower contents of alkalis and alumina (12–14 wt % Al₂O₃) and low contents of granitophile elements (Rb, Li, Cs, Be, Nb, Ta, and W), Cr, and Ni. The Cr/V (<0.10) and Rb/Sr (0.01–0.1) ratios of these rocks are at a minimum, and their K/Rb (600–1000) and Na/K (5–10) ratios are at a maximum compared to those of the rocks of the most widely spread granitoid batholiths. The plagiogranites typically have REE concentrations higher than those in oceanic plagiogranites and display weakly fractionated REE patterns (La/Yb = 1.4–3.4) with weak (or without) Eu anomalies. The lower initial Sr ratios of these rocks (0.704) and their relatively high concentrations of Pb, Zr, and B testify to the predominantly mantle provenance of their protolithic material. Geological and geochemical characteristics of the Yenisei pluton suggest that its genesis can be considered within the scope of the model of retrograde-type magmatic replacement and that the batholith was produces by the earliest granitization processes in the oceanic crust. The granitic melt was derived at low pressures (<5 kbar) and intermediate temperatures (～700°C), at the inflow of an aqueous transmagmatic fluid into the magma-generating area and the subsequent fluid-magmatic differentiation. Considering the volumes and compositions of rocks composing the Yenisei Batholith, the latter can be attributed, similarly to other typical granitoid batholiths, to crustal plutons, which differ from both oceanic plagiogranites in ophiolitic belts and continental trondhjemites. The rocks can be regarded as an individual geochemical type of crustal plagiogranites.     

Pan African layered diorite-tonalite-granodiorite from Sinai massif: an open system fractionation at a subduction zone

This paper reports on the occurrence of layered Pan African dioritetonalite-granodiorite (DTG) rocks. The layering is marked by alternation of melanocratic (M) layers (diorites and tonalites) and leucocratic (L) layers (tonalites and granodiorites). M-samples have cumulus biotite+hornblende+relict pyroxene+plagioclase+K-feldspars+magnetite+apatite, and have transitional calc-alkaline and metaluminous affinities. They were derived from subduction-related magma enriched in Rb, Ba, K and LREE and depleted in Sr and Nb. L-samples have cumulus plagioclase+hornblende. They are enriched in Sr and depleted in Rb, Ba, K, Nb and LREE. They have calc-alkaline and peraluminous affinites.
The formation of the rhythmic layers of DTG composition can be attributed to periodical replenishment of pulses of basic magma into a more evolved acidic magma chamber under open system conditions. Field relations, mineralogy and element concentration among the M- and L-layers indicate that at the subduction zone, the ascending magma was contaminated with lower crustal materials (marginal basin metasediments) which led to LILE-enrichment, Nb-depletion and transition from calc-alkaline to alkaline and from metaluminous to peraluminous affinities as well.     

The geochemistry of Precambrian bedrocks

The study of the geochemistry of trace elements in magmatic and ultrametamorphic rocks permits the characterization of geochemical types of granitoids and basites that differ in their mode of formation, major chemical, trace-element and mineral composition, and potential as indicators of ore bodies.For 12 geochemical types of granites the ratio F(Li + Rb)/(Sr + Ba) is characteristic and may change by more than 3 orders of magnitude.For 8 geochemical types of basites the most informative ratio is K(Sr + Ba)/(Ni + Cr), the value of which may change within 5 orders of magnitude.The geochemical typification of magmatites and ultrametamorphites permits consideration of the geochemical particulars of the formation of the Precambrian rocks of the continental Earth's crust.The formation of the continental crust, hydrosphere and atmosphere (3.8−1.6 Ga), and the formation of the granitic cores of Archean cratons, apparently resulted from a process of ultrametamorphism and granitization of a basic protocrust and proceeded under the influence of intratelluric diffusions, originating as a result of the degassing of the upper mantle.During the initial stages of the process (formation of plagio-migmatites and enderbites from the rocks of the protocrust) a marked loss of Mg, Ca, Fe and a series of trace elements (Ni, Au, etc.) took place. Later during the formation of K-feldspar migmatites, charnockites and true ultrametamorphic granites, a marked increase of K, Rb, Ba and a series of other trace elements took place. During the final stage, marked by an intensive influx of F, (rapakivi) granite and prefault rare-metal metasomatites were formed.During the formation of greenstone belts, the second important feature of Archean cratons, a process of penetration, differentiation, crystallization and degassing of silicate melts of ultrabasic and basic composition, resulting from the selective melting of parts of the upper mantle, predominated. Later sedimentary-metamorphic strata also formed in these belts.The peculiarities of the processes of the formation of the Precambrian continental crust predetermined the metallogenic characteristics of this important and long stage of the Earth's development, characterized by the formation of enormous commercial concentrations of Fe, Ni, Au and a series of other elements.     

东昆仑西大滩混合岩带的基本特征及其成因初探

研究沿昆仑山口北侧大致呈东西向展布的西大滩混合岩带,对探讨东昆仑地区的构造演化,厘定东昆仑地体和巴颜喀拉地体的边界性质,有着十分重要的意义。为此,我们在开展青藏高原地学断面研究之际,横穿东昆仑构造带,进行了近南北方向的地质调查,重点对西大滩三道沟混合岩进行变质构造研究,以期对昆仑地质构造演化提供必要的实际素材。     

Early Eocene magmatism in the Sredinnyi Range,Kamchatka: Composition and geodynamic aspects

Migmatization and granite-forming processes were widespread in the southern Sredinnyi Range of the Kamchatka Peninsula in the Early Eocene (at approximately 52 ± 2 Ma). The paper presents data on the composition and genesis of the Early Eocene granitoids. The Malka Rise contains both equigranular peraluminous garnet-bearing granites, on the one hand, and migmatites and tonalites and trondhjemites (TTG), on the other. The petrography and petrochemistry of most granites in the Malka Rise in the Sredinnyi Range (high SiO₂ concentrations, the presence of muscovite and garnet, the proportions of their Al saturation index ASI and SiO₂, FeO_t + MgO + TiO₂, and SiO₂, Al₂O₃/TiO₂, and CaO/Na₂O), and the composition of biotite in these rocks highlight their similarities with S-granites. The character of the REE patterns and the Sr and Y concentrations suggest that the granites and TTG were formed via the melting of sources of two types: metasediments and metabasites. The metasedimentary nature of the protolith of most of the granitoids also follows from similarities between the REE patterns of the granitoids and host metaterrigenous rocks of the Kolpakova and Kamchatka groups. The variations in the Rb/Ba and Rb/Sr ratios of the granites imply that their protoliths could be sedimentary rocks both depleted and enriched in pelite components. The facts that, along with S-granites, some of the granites are TTG, which likely had mafic protoliths, make the Early Eocene granites generally similar to S-granites of the Cordilleran type. The collision of the Achaivayam-Valaginskii ensimatic island arc with the Kamchatka margin of Eurasia started at 55–53 Ma and predated Early Eocene magmatism. In the course of this collision, arc complexes were obducted over continental marginal rocks, and this resulted in their rapid subsidence, crustal heating, magma generation, and the derivation of the granites, tonalites, and trondhjemites at 52 ± 2 Ma at temperatures of 645–815°C. This rapid heating (duirng no more than 3–5 Ma) required an additional heat source, which was likely the mantle. The latter heated the bottom of the crust at the detachment of the slab. The influx of mantle material resulted in intrusions of the norite-cortlandite association, which was coeval with the granites and was accompanied by Cu-Ni sulfide mineralization. The composition of the granitoids and data on the intrusions of the norite-cortlandite association suggest that mantle material was involved in Early Eocene syncollisional magma generation in Kamchatka. Newly obtained U-Pb zircon SHRIMP dates of the granitoids and recently published data on the age of the norite-cortlandite intrusions indicate that they are coeval and make it possible to recognize an Early Eocene phase of magmatic activity in Kamchatka.     

Geochemistry of an island-arc plutonic suite: Wadi Dabr intrusive complex, Eastern Desert, Egypt

The Wadi Dabr intrusive complex, west of Mersa-Alam, Eastern Desert, Egypt ranges in composition from gabbro to diorite, quartz diorite and tonalite. The gabbroic rocks include pyroxene-horn blend e gabbro, hornblende gabbro, quartz-hornblende gabbro, metagabbro and amphibolite. Mineral chemistry data for the gabbroic rocks indicate that the composition of clinopyroxenes ranges from diopside to augite and the corresponding magma is equivalent to a volcanic-arc basalt. Plagioclase cores range from An₇₅ to An₃₄ for the gabbroic varieties, except for the metagabbro which has An _11–18. The brown amphiboles are primary phases and classified as calcic amphiboles, which range from tschermakitic hornblende to magnesiohornblende. Green hornblende and actinolite are secondary phases. Hornblende barometry and hornblende-plagioclase themometry for the gabbroic rocks estimate crystallisation conditions of 2–5 kb and 885–716°C.The intrusive rocks cover an extensive silica range (47.86–72.54 wt%) and do not exhibit simple straight-line variation on Harker diagrams for many elements (e.g. TiO₂, Al₂O₃, FeO^*, MgP, CaO, P₂O₅, Cr, Ni, V, Sr, Zr and Y). Most of these elements exhibit two geochemical trends suggesting two magma sources.The gabbroic rocks are relatively enriched in large ion lithophile elements (K, Rb, Sr and Ba) and depleted in high field strength elements (Nb, Zr, Ti and Y) which suggest subduction-related magma. Rare earth element (REE) data demonstrate that the gabbroic rocks have a slight enrichment of light REE [(La/Yb)_N=2.67−3.91] and depletion of heavy REE ((Tb/Yb)_N=1.42−1.47], which suggest the parent magma was of relatively primitive mantle source.The diorites and tonalites are clearly calc-alkaline and have negative anomalies of Nb, Zr, and Y which also suggest subduction-related magma. They are related to continental trondhjemites in terms of Rb---Sr, K---Na---Ca, and to volcanic-arc granites in terms of Rb---and Nb---Y.The Wadi Dabr intrusive complex is analogous to intrusions emplaced in immature ensimatic island-arcs and represents a mixture of mantle (gabbroic rocks) and crustal fusion products (diorites and tonalites) modified by fractional processes.     

Chemical characteristics and genesis of the quartz-feldspathic rocks in the Archean crust of Greenland

A total of 108 samples of meta-tonalites, metagranodiorites, granites and meta-tholeiites representing groups of Early to Late Archean age and different metamorphic history from SW and SE Greenland have been analyzed for Ca, K and 28 trace elements. There is no systematic change of the chemical composition with age observable. The results support petrologic experiments which suggest that tonalites and granodiorites (the most abundant rocks of the Archean crust) are partial melting products of a mafic lower crust. Modelling suggest that this crust consisted of garnet amphibolite derived from a source with a bulk composition resembling a slightly enriched rather than depleted mantle. The Ce_N/Yb_N ratio is above 10 in the majority of tonalites. Most samples have no Eu anomaly because of a balanced contribution from the minerals of a mafic rock (or a plagioclase-free source). The positive Eu anomaly of some granodiorites and of a minor proportion of tonalites can be explained as being caused by plagioclase accumulation during differentiation or by partial melting of plagioclase-rich fractions. Modelling with Zn excludes an origin of tonalitic melts by differentiation of basaltic to dioritic magmas. The Archean meta-diorites, meta-tonalites and meta-granodiorites from Greenland have generally lost some K and S relative to their suggested magmatic protoliths. Loss of Rb, Tl, Pb and K and relative gain of Ca, Sr, Ba and Sc connected with granulitization of meta-tonalites can be explained in the majority of cases by separation of about 25 percent granitic partial melt. High K/Rb, K/Pb, Zn/Cd and Nb/Th ratios of granulites plus low ratios of granites are almost in balance with intermediate ratios of amphibolite-facies tonalites. Retrogression of granulites into amphibolites was accompanied by introduction of Pb, Tl, Rb, Ba, Sr and K from Na-Cl-rich brines circulating on fractures. A comparison of the abundance of 24 elements (characterized by different compatibility) in the Archean crust of Greenland with the present bulk crust reflects only minor changes (Th, Nb) if at-all in the chemical composition of the continental crust since the Archean.     

Metacarbonate Rocks (Calciphyres) of the Lapland—Kolvitsa Granulite Belt, Baltic Shield

Geological, petrochemical, and geochemical data are presented on metacarbonate rocks (calciphyres) of the Lapland—Kolvitsa granulite belt, Baltic Shield. The normative mineral composition of source rocks of the studied calciphyres was first reconstructed. High contents of some indicator elements (Fe, Mn, Cr, Co, P, Pb, and others) suggest that material supplied to the paleobasin was derived from diverse rocks (ultramafic, mafic, intermediate, and felsic). Contents and correlations of some trace elements (Sr, Li, F, Ba, and others) indicate that primary sediments formed under humid-semihumid paleoclimate in a fresh-water paleobasin (lagoon) characterized by occasional increase in salinity.     

Early Precambrian tonalite-trondhjemite sialic nuclei

Early Precambrian batholiths evolved by diapiric intrusion of near-liquidus to superheated tonalitic and trondhjemitic magmas into an early greenstones crust. Distribution patterns of enclaves and xenolith screens derived from the latter provide reference markers which define the internal geometry and detailed structure of the “gregarious batholiths” (Macgregor) as polydomal multi-lobal bodies. Near-liquidus temperatures are suggested by the digestion of vast volumes of ultramafic—mafic crust by the acid magmas. Tracing of xenolith trains between low and high grade metamorphic terrains provide key evidence for coeval relations between granite—greenstone type terrains and amphibolite to granulite facies infracrustal root zones of the latter. The formation of the plutonic tonalite—trondhjemite suite was accompanied by dacitic to rhyolitic extrusions, the acid volcanic lenses being located above early greenstone units intruded by the batholiths and below upper greenstone sequences which postdate these intrusions. The geochemical characteristics of high-level and deep-level tonalites and trondhjemites are compared. Both suites display very wide compositional spectra, but data from high-grade terrains tend to define a more basic field than data from granite—greenstone terrains. Effects of source compositions on the geochemistry of the acid plutonic rocks are pointed out. Tonalites dominate in South African terrains whereas trondhjemites dominate in Western Australian terrains — a difference conceivably related to the more ultramafic composition of source rocks represented by early greenstone units in southern Africa. Granodiorites and potassic granites form a comparatively minor component of Archaean batholiths, and may occur in the following forms: (1) bands of augen gneiss in high-grade terrains; (2) components of trondhjemitic to granitic gneisses in high-level plutons; and (3) discrete post-tectonic intrusions typically emplaced at high levels of the batholiths and along older tonalite—greenstone contacts. Migmatites characteristically form in close spatial association with xenolith-rich zones, probably due to depression of the solidus consequent on water addition related to dehydration of the xenoliths. A derivation of the acid sodic magmas by anatexis of sialic materials is inconsistent with geochemical evidence and petrological theory. In contrast, the commonly low to very low LIL element levels and REE evidence indicate derivation by about 30–50% melting of basic rocks. Marked trace element anomalies are characteristic of some Archaean plutonic suites, e.g. very high Sr in some Western Australian rocks, low Rb in some Lewisian (Scotland) and South African rocks, U depletion in South African and southwestern Greenland suites, high Li in some Pilbara rocks and high Zr in some southwestern Greenland rocks. However, the only consistent anomaly observed to date is a well-pronounced depletion in Y and heavy REE, suggesting extensive equilibration of the acid melts with eclogite and/or amphibolite. Uniformitarian interpretations of the Archaean are questioned in the light of the evidence for high temperature and pressure, the unique tectonic style of diapirism and the low initial⁸⁷Sr/⁸⁶Sr as compared to Proterozoic plutonic suites. The diachronous nucleation of tonalite—trondhjemite plutons during the Archaean is seen as the major process effecting a transformation of an early Archaean sima into sial.     

Recycled oceanic crust as a source for tonalite intrusions in the mantle section of the Khor Fakkan block,Semail ophiolite (UAE)

Several types of felsic granitoid rocks have been recognized, intrusive in both the mantle and the crustal sequence of the Semail ophiolite. Several models have been proposed for the source of this suite of tonalites, granodiorites, trondhjemites intrusions, however their genesis is still not clearly understood. The sampled Dadnah tonalites that intruded in the mantle section of the Semail ophiolite display arc-type geochemical characteristics, are high siliceous, low-potassic, metaluminous to weakly peraluminous, enriched in LILE, show positive peaks for Ba, Pb, Eu, negative troughs for U, Ti and occur with low δ¹⁸O_H2O, moderate ε_Sr and negative ε_Nd values. They have crystallized at temperatures that range from ∼550 °C to ∼720 °C and pressure ranging from 4.4 kbar to 6.5 kbar. The isotopic ages from our tonalite samples range between 98.6 Ma and 94.9 Ma, slightly older and overlapping with the age of the metamorphic sole. Our field observations, mineralogical, petrological, geochemical, isotopic and melt inclusion data suggest that the Dadnah tonalites formed by partial melting (∼10%–15% continuous or ∼12% batch partial melting), accumulation of plagioclase, fractional crystallization (∼55%–57%), and interaction with their host harzburgites. These tonalites were the end result of partial melting and subsequent contamination and mixing of ∼4% oceanic sediments with ∼96% oceanic lithosphere from the subducted slab. This MORB-type slab melt composed from ∼97% recycled oceanic crust and ∼3% of the overlying mantle.We suggest that a possible protolith for these tonalites was the basaltic lavas from the subducted oceanic slab that melted during the initial stages of the supra-subduction zone (SSZ), which was forming synchronously to the spreading ridge axis. The tonalite melts mildly modified due to low degree of mixing and interaction with the overlying lithospheric mantle. Subsequently, the Dadnah tonalites emplaced at the upper part of the mantle sequence of the Semail ophiolite and are geochemically distinct from the other mantle intrusive felsic granitoids to the south.     

Late Paleozoic gabbroic rocks of the Bridge River accretionary complex,southwestern British Columbia: geology and geochemistry

Gabbroic bodies in the Bralorne-Gold Bridge area of southwestern British Columbia are associated with the oceanic Bridge River complex of the western Canadian Cordillera, one of the suspect terranes accreted to North America in the Jurassic. The gabbros are locally cut by tonalites and are structurally interleaved with ultramafic rocks, phyllites, graphitic cherts, and carbonate lenses that comprise the lower part of the Bridge River complex. Their late Carboniferous crystallization age overlaps the depositional age of affiliated supracrustal rocks (Mississippian-Jurassic), some of which have been metamorphosed to blueschist facies. Compositionally, the gabbros resemble mafic plutonic rocks of ophiolitic complexes and gabbroic rocks of the nearby Shulaps Range. They display some affinity to oceanic island arc tholeiitic suites. The Bralorne and Shulaps gabbros include cumulates and appear to have been derived from a single, light REE-depleted, peridotitic source by melting and subsequent fractional crystallization/accumulation of various combinations of plagioclase, pyroxenes, and olivine. The tonalites are compositionally distinct from typical ophiolitic plagiogranites, but might be related to the associated gabbros. The gabbroic bodies occur within tectonic slivers derived from the oceanic crust that floored a deep ocean basin that existed during the late Paleozoic and early Mesozoic. The Bridge River complex comprises fragments of oceanic crust that were tectonically incorporated into an east-verging accretionary prism during a middle/late Triassic to Jurassic collisional event.     

辽东半岛东南部南辽河群变质火山岩的时代、成因及其对区域构造演化的制约

辽东半岛东南部地区出露大面积南辽河群变质表壳岩系,其下部尤以广泛发育变质火山岩为特征。本文通过对黑云斜长片麻岩、斜长角闪岩以及黑云母变粒岩等岩石系统的岩石学、全岩地球化学研究,并结合锆石U-Pb年代学以及Lu-Hf的研究结果来制约其物质组成、原岩形成和变质时代、岩石成因及形成环境,进而探讨胶—辽—吉活动带的大地构造属性。锆石显微结构、微区成分及LA-ICP-MS U-Pb年代学分析结果表明,研究区南辽河群变质火山岩原岩可能形成于2.19Ga左右,并记录了1.90Ga左右的变质事件。元素和同位素地球化学分析结果表明,其原岩以玄武安山岩-安山岩和英安岩及少量的流纹岩为主,主体属于亚碱性系列,具有钙碱性演化趋势。其中,基性端元以相对较低的SiO_2质量分数,富集MgO、TFe_2O_3和轻稀土元素(LREE)及Cr、Co和Ni为特征,强烈亏损重稀土元素(HREE)和高场强元素(HFSE,如Nb、Ta和Ti),岩浆应起源于受俯冲流体或熔体交代的亏损岩石圈地幔楔;而中—酸性端元则具有高的SiO_2质量分数和较低的MgO、TFe_2O_3质量分数,富碱,富集Ba、Th、U和K,以及亏损Nb、Ta、P和Ti等特征,应来自于新生地壳物质的部分熔融。结合区域地质资料表明,南辽河群变质火山岩应形成于一个典型的活动大陆边缘的构造环境,暗示胶—辽—吉活动带北段古元古代中期以前的形成演化机制应与弧-陆碰撞作用有关。     

Elemental and Sr–Nd isotopic compositional disparity of riverine sediments around the Yellow Sea: Constraints from grain-size and chemical partitioning

To obtain a better understanding of the source compositions of the river sediments around the Yellow Sea and their relationship with source rocks, elements and strontium-neodymium (Sr–Nd) isotopes of different grain-sizes (silt and clay populations) and chemical (labile and residual phases) fractionations in riverine sediments were studied extensively. These results clearly revealed a systematic compositional disparity between Korean river (KR) and Chinese river (CR) sediments, especially in the residual (detrital) fraction. The geochemical dissimilarity between these might reflect inherited signatures of their source rocks but with minor control from chemical weathering. In particular, the remarkable enrichment of some elements (iron (Fe) and magnesium (Mg)) and the behavior of large ion lithophile elements (e.g., barium (Ba), potassium (K) and Sr) during weathering as well as less-radiogenic Sr isotopic compositions implies that CR sediments might be weathering products of relatively more mafic rocks, with abundant ferromagnesian and plagioclase feldspar minerals, compared with KR sediments derived from silicic granites with relatively higher quartz and potassium feldspar contents. This different petrological rationale is clearly evident in an A–CN–K diagram, which estimated the source rock of CR sediments as granodioritic, a composition that reflects accurately the average composition of weathered continental crust in China. The recognition of such geochemical systematics in two river sediments, especially in grain-size and chemically partitioned data, may contribute to the establishment of provenance tracers for the Yellow Sea and East China Sea sediments with multi-sources as well the dust deposition in the western Pacific.