The Coastal Accretionary Complex of central Chile constitutes the product of early Carboniferous to Late Triassic subduction at the rear of Chilenia, a continental terrane likely derived from Laurentia and accreted to southwestern margin of Gondwana during the Mid to Late Devonian. The complex contains basaltic metavolcanic sequences of the subducted oceanic lithosphere accreted to the active margin. In this paper, we address the tectonic setting of these rocks by means of a geochemical study in the coastal area of Pichilemu region, central Chile. The accreted fragments of oceanic crust occupy different structural levels, exhibit variable metamorphic grade, and have geochemical fingerprints that reveal a compositional heterogeneity of the subducted oceanic crust. The amphibolites have N to E-MORB compositions. Greenschist units include N-MORB and E-MORB transitional to OIB, and blueschists and greenschists interleaved within a single metavolcanosedimentary sequence have OIB signatures. Neodymium isotopic systematics indicate depleted and enriched mantle sources, whereas strontium isotopic systematics indicate seawater/rock interaction. The variety of rocks suggests formation in an oceanic setting characterized by shallow and deep mantle sources, such as plume-influenced ridge. Based on the geological, petrological, geochemical, and isotopic characteristics, we propose that the metavolcanic protoliths of the Pichilemu region formed relatively close to the western margin of the Chilenia terrane during the initial stage (late Cambrian–Early Devonian) of seafloor development and drifting of this continental block. Geochemical similarities with oceanic units accreted to the active margin south of the Pichilemu region indicate a regional pattern of the oceanic crust subducted under the Palaeozoic Chilean margin between, at least, 34°S and 39°S latitude, strongly supporting the activity of a mantle plume. This, in turn, can be correlated with the location of the Pacific plume generation zone in early Palaeozoic era, corroborating a Laurentian origin for the Chilenia terrane. 相似文献
In the Upper Murray Valley, Victoria, Late Silurian, high‐Si igneous rocks, which are closely associated with alkalic, basaltic dykes, were emplaced at high crustal levels following the peak of the Benambran Orogeny, which deformed and metamorphosed the Wagga Zone in Late Ordovician‐Early Silurian times. These rocks, which are informally termed ‘the Upper Murray high‐Si magmatic suite’, include leucogranites, rhyolite dykes and flows, and ash‐flow tuffs characterised by the following features. They are transitional from mildly peraluminous to mildly metaluminous; they represent relatively anhydrous magmas, in which halides were important volatile constituents; they have high Si, total alkalies, Rb, Th, U, Nb, Sn and heavy rare earth elements; and they are relatively repleted in Mg, Ca, Sr, Eu, V, Cr and Ni. In these respects and in their post‐orogenic setting and close association with alkalic basalts, they resemble many post‐orogenic granitoids from elsewhere. Such granitoids appear to have formed as partial melts during crustal extension following major episodes of deformation and high‐Si magmatism. A residual granulitic crust, from which an earlier generation of granitoid magmas had been extracted, is argued to be the source rock‐type for these post‐orogenic magmas. Tectonic extension, affecting such a crust, was accompanied by deep fracturing and basaltic vol‐canism. Mantle‐derived, CO2‐ and halide‐rich fluids moved into the residual crust, causing widespread metasomatism, and emplacement of basaltic magma caused temperatures to rise until melting took place and a second group of magmas was produced. This model explains most aspects of the trace and major element chemistry of post‐orogenic, high‐Si igneous rocks and, for the Upper Murray high‐Si suite it also provides an explanation for variations in trace elements and isotopic characteristics. Other processes, such as crystal fractionation, magma mixing, thermogravi‐tational diffusion, and separation and loss of a volatile phase, provide explanations for variations within individual units of the suite, but they do not explain overall variations or the highly fractionated nature of the suite. 相似文献
The Sarek Dyke Swarm (SDS) crops out in the Sarektjåkkå Nappe (SN) of the Seve-Kalak Superterrane in the northern Swedish Caledonides. The SN has two main components: (1) a 4–5 km thick succession of rift-related sedimentary rocks, which is intruded by (2) a suite of tholeiitic dykes (the SDS) constituting 70–80% of the nappe. The nappe was deformed during Caledonian thrusting, but dykes and sedimentary rocks in the interior of the eastern parts of the SN are preserved in a pristine state. The tholeiitic dykes of the SDS commonly occur in sheeted dyke complexes, and up to 11 successive generations can be identified from crosscutting relations. The SN represents the fossil continent–ocean transition between the Baltic craton and the Iapetus Ocean, marking the initiation of seafloor spreading. Bubble-shaped pods and veinlets of diorite are present in the SDS sheeted dyke complexes. The pods are absent in the oldest dykes, but the younger a dyke, the more frequent the pods. The diorite pods are the equivalent of gabbro pegmatites, and both cogenetic and coeval with the dykes. The rapid successive emplacement of tholeiitic magma raised the ambient temperature in the dyke complex, so that crystallization in the youngest dykes mimicked similar processes in gabbro plutons. Six zircon fractions, from the diorite pods including two single grains, were analysed geochronologically by the U–Pb thermal ionization mass spectrometry method. The data yield a linear array of points that are 0.4–0.8% normally discordant, indicating a crystallization age of 608±1 Ma (207Pb/206Pb=607.9±0.7 Ma, MSWD=0.33). This age is inferred to date the onset of seafloor spreading in the Iapetus Ocean along the Baltoscandian margin. 相似文献
The Chalukou porphyry Mo deposit, located in the Great Hinggan Range, is the largest Mo deposit in northeast China, although the age and genesis of the associated magmatic intrusions remain debated.Here we report zircon U-Pb ages and trace elements, whole rock geochemistry and Sre Nd isotope data with a view to understand the relationship between the magmatism and molybdenum mineralization.Zircon U-Pb analysis yield an age of 475 Ma for rhyolite in the older strata, 168 Ma for the premineralization monzogranite, and 154 Ma for the syn-mineralization granite porphyry. The granite porphyry and quartz porphyry are considered as the ore-forming intrusions. These rocks are peraluminous, alkali-calcic, and belong to high-K to shoshonitic series with a strong depletion of Eu. They also display characteristics of I-type granites. The rocks exhibit wide variations of(87 Sr/86 Sr)iin the range of 0.705426 -0.707363, and ε_(Nd)(t) of -3.7 to 0.93. Zircon REE distribution patterns show characteristics between crust and the mantle, implying magma genesis through crust-mantle interaction. The Fe_2O_3/FeO values(average 1) for the whole rock and EuN/Eu*Nvalues(average 0.45), Ce~(4+)/Ce~(3+) values(average 301)for zircon grains from the granite porphyry are higher than those from other lithologies. These features suggest that the ore-forming intrusions(syn-mineralization porphyry) had higher oxygen fugacity conditions than those of the pre-mineralization and post-mineralization rocks. The Chalukou Mo deposit formed in relation to the southward subduction of the Mongol-Okhotsk Ocean. Our study suggests that the subduction-related setting, crust-mantle interaction, and the large-scale magmatic intrusion were favorable factors to generate the super-large Mo deposits in this area. 相似文献