Abstract Arc magmas form new continental crust and are responsible for volcanic eruptions as well as for major metallic ore deposits. It is generally accepted that arc magmas are generated above subduction zones by partial melting of the mantle wedge and differentiate within the crust of the overriding plate through fractional crystallization, magma mixing and crustal assimilation. However, it is not clear in which proportions mantle and the above different intracrustal processes contribute to the broad geochemical variability of arc magmas. Here, using Nd isotope systematics and their geochemical modelling, I show that the thicker the crust of the overriding plate, the higher the assimilation rate of crustal rocks by mantle-derived magmas and the older the assimilated rocks. This highlights a systematic increase of crustal contribution to arc magma chemical and isotopic composition with the thickening of the overriding plate crust. The data presented are also consistent with growth and maturation of the continental crust through time by continuously increasing thickness, SiO 2 content and Nd isotopically evolved composition.
Abstract Post-collisional ultrapotassic rocks (UPRs) in the Tibetan Plateau exhibit extreme enrichment in incompatible elements and radiogenic isotopes. Such enrichment is considered to be either inherited from a mantle source or developed during crustal evolution. In this study, to solve this debate we combined mineral textures and in situ geochemical composition of clinopyroxene phenocrysts in UPRs from southern Tibet to reveal their crustal evolution, enrichment cause and constrain metasomatism in their mantle source. Results show that the UPRs experienced an array of crustal processes, i.e., fractional crystallization, mixing, and assimilation. Fractional crystallization is indicated by decreases in Mg# and Ni and enrichment in incompatible elements (e.g. rare earth element (REE), Sr, Zr) toward the rims of normally zoned clinopyroxene phenocrysts (type-I). Magma mixing is evidenced by the presence of some clinopyroxene phenocrysts (type-II, -III) showing disequilibrium textures (e.g. reversed and overgrowth zoning), but in situ Sr isotope and trace element analysis of those disequilibrium zones indicate that late-stage recharged mafic magmas are depleted (87Sr/86Sr: 0.70659–0.71977) compared with the primitive ultrapotassic magmas (87Sr/86Sr: 0.70929–0.72553). Assimilation is revealed by the common presence of crustal xenoliths in southern Tibetan UPRs. Considering the much lower 87Sr/86Sr values (0.707759–0.709718) and incompatible element contents of these crustal xenoliths relative to their host UPRs, assimilation should have resulted in geochemical depletion of southern Tibetan UPRs rather than enrichment. The diluting impact of both assimilation and mixing is also supported by the modeling results based on the EC-E′RAχFC model combining the growth history of clinopyroxene. Trace elements ratios in clinopyroxenes also imply that the mantle source of southern Tibetan UPRs suffered an enriched and carbonatite-dominated metasomatism. Thus, we conclude that enrichment of southern Tibetan UPRs was inherited from the mantle source.
Triassic orthogneisses of the Antarctic Peninsula provide evidence for the Palaeozoic and Mesozoic geological evolution of southern Gondwana within Pangaea. These rocks are sporadically exposed in southeastern Graham Land and northwestern Palmer Land, although reliable geochronological, geochemical and isotopic data are sparse. We combine new geochronological (LA-ICP-MS zircon U-Pb), geochemical, and zircon (Hf, O) and whole rock isotopic (Nd, Sr and Pb) data to constrain the age and Triassic – Palaeozoic tectonic setting of these rocks. Zircon cores record a Palaeozoic magmatic arc between 252.5 ± 2 and 527.8 ± 6.2 Ma, which was mainly located to the west of the Eastern Palmer Land Shear Zone (Central Domain; Vaughan and Storey, 2000). The arc is considered to be an extension of contemporaneous Palaeozoic arcs that have been identified along the Pacific margin of South America and the Thurston Island Block. Regions to the east of the Palmer Land Shear Zone (Eastern Domain, Vaughan and Storey, 2000) were located distal from the Terra Australis Margin, and possibly resided within Sunsas-aged belts within Pangaea. Triassic continental arc, calc-alkaline magmatism during 223.4 – 203.3 Ma modified the crust of the Antarctic Peninsula on both sides of the Eastern Palmer Land Shear Zone. Magmatic sources included igneous and sedimentary crustal material, which formed by crustal reworking during Sunsas- and Braziliano-aged orogenesis, and Palaeozoic arc magmatism. Arc magmatism accompanied sinistral extension which brought both domains into the arc and resulted in steady oceanward migration of the Triassic arc during the Middle – Late Triassic. We conclude that the Eastern Palmer Land Shear Zone formed in the Triassic, and that both the Eastern and Central Domains are autochthonous to Gondwana.
The Oligocene magmatism in the Eastern Pontides (EPs) and the Tauride-Anatolide block (TAB) to its south remains poorly understood because of limited outcrops. Rare Oligocene dikes, documented during mapping in the Yanıklı prospect within the Artvin district of the easternmost EPs, offer us a unique opportunity to investigate Oligocene magmatism in this region of the Tethyan orogenic belt. Three different NW-oriented Oligocene dikes have been identified and characterized by their composition and age. One dike type named ODike-1 has yielded U-Pb zircon ages of 31.04±0.19 Ma, and exhibits non-adakitic compositions, Two other dike types with adakite-like characteristics, named ODike-2 and ODike-3, have yielded, respectively, U-Pb zircon ages of 31.05±0.20 to 29.73±0.31 Ma and 29.21±0.31 Ma.These dikes originated from the low-degree partial melting of a thickened juvenile lower crust during asthenospheric mantle upwelling. 143Nd/144Ndi and 87Sr/86Sri ratios, respectively, of 0.512742-0.512784 and 0.704714-0.706164 indicate the assimilation of an asthenospheric melt during the upwelling process. We conclude that the asthenospheric upwelling was a significant heat source, while the lower portion of the delaminated crust continued sinking vertically into the underlying mantle. This initiated partial melting of the overlying lithospheric mantle during the Oligocene in Yanıklı. Additionally, the rollback of the Southern Neotethys during the Oligocene may have provided additional heat flow, influencing magmatic activity in the EPs to the north.Comparisons between the Oligocene magmatism in Yanıklı and other regions in the TAB and South Armenian block (SAB) reveal distinct characteristics, suggesting that the magmatic activity in Yanıklı is not directly linked to the same tectonic history observed in the TAB and SAB regions. This study significantly contributes to our understanding of magmatic processes and tectonic dynamics in the EPs, thereby advancing the geotectonic framework of the central Tethyan orogenic belt.
Abstract Intermediate-sulfidation epithermal deposits are important not only for their significance in terms of Ag endowment but also for their close link with porphyry deposits. There are two subtypes of intermediate-sulfidation deposits, namely Au-rich (Ag/Au <60) and Ag-rich (Ag/Au >60). The tectonomagmatic factors that control the formation of either one of the two types have not been thoroughly examined. Here we investigated the degree of magmatic fractionation, magma oxygen fugacity, and F-Cl-S concentrations in the magmas, as well as their sources, for the magmatic rocks associated with four intermediate-sulfidation deposits (two Ag-rich: Baiyinchagan, Shuangjianzishan; two Au-rich: Naozhi, Zhengguang; all in northeast China) and found that the ore-forming magmas of our two Au-rich deposits in northeast China belong to the I type, whereas those of our two Ag-rich deposits are of the A type. Ore-forming magmas of our Au-rich intermediate-sulfidation deposits belong to the magnetite series and are less evolved, slightly more juvenile, more oxidized (ΔFMQ = 0.5 to 1.5; FMQ = fayalite-magnetite-quartz), more Cl rich, and more hydrous than magmas associated with the Ag-rich deposits. These features are typical of neutral-compressional thick magmatic arcs. By contrast, the ore-forming magmas of our two Ag-rich deposits belong to the ilmenite series and are more evolved, reduced (ΔFMQ <0.5), moderately juvenile, more F rich, and less hydrous. They likely originated from prolonged differentiation of juvenile crust with heterogeneous and minor addition of sediment melts. These features are typical of a back-arc extensional regime developed within accreted juvenile arcs in northeast China. These findings provide a first-order framework for guiding early-stage porphyry-epithermal exploration of untested prospects.
Abstract The volcanic rocks of the Chon Aike Silicic Large Igneous Province (CASP) are recognized as magmas dominantly produced by crustal anatexis. Investigating the zircon of the CASP provides an opportunity to gain further insight into geochemical and isotopic differences of the potential magmatic sources (i.e., crust versus mantle), to identify crustal reservoirs that contributed to the felsic magmas during anatexis, and to quantify the contributions of the respective sources. We present a combined zircon oxygen and hafnium isotope and trace element dataset for 16 volcanic units of the two youngest volcanic phases in Patagonia, dated here with LA-ICP-MS U–Pb geochronology at ca. 148–153 Ma (El Quemado Complex, EQC) and ca. 159 Ma (western Chon Aike Formation, WCA). The EQC zircon have 18 O-enriched values (δ 18 O from 7 to 9.5‰) with correspondingly negative initial εHf values (− 2.0 to − 8.0). The WCA zircon have δ 18 O values between 6 and 7‰ and εHf values ranging between − 4.0 and + 1.5. Binary δ 18 O-εHf mixing models require an average of 70 and 60% melt derived from partial melting of isotopically distinct metasedimentary basements for the EQC and WCA, respectively. Zircon trace element compositions are consistent with anatexis of sedimentary protoliths derived from LIL-depleted upper continental crustal sources. The overlap between a high heat flux environment (i.e., widespread extension and lithospheric thinning) during supercontinental breakup and a fertile metasedimentary crust was key in producing voluminous felsic volcanism via anatexis following the injection and emplacement of basaltic magmas into the lower crust.
Porphyry systems supply most copper and significant gold to our economy. Recent studies indicate that they are frequently associated with high Sr/Y magmatic rocks, but the meaning of this association remains elusive. Understanding the association between high Sr/Y magmatic rocks and porphyry-type deposits is essential to develop genetic models that can be used for exploration purposes. Here we present results on a Pleistocene volcano of Ecuador that highlight the behaviour of copper in magmas with variable (but generally high) Sr/Y values. We provide indirect evidence for Cu partitioning into a fluid phase exsolved at depths of ~15 km from high Sr/Y (>70) andesitic magmas before sulphide saturation. This lends support to the hypothesis that large amounts of Cu- and S-bearing fluids can be accumulated into and released from a long-lived high Sr/Y deep andesitic reservoir to a shallower magmatic-hydrothermal system with the potential of generating large porphyry-type deposits.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)