Relative contributions of crust and mantle to generation of Campanian high-K calc-alkaline I-type granitoids in a subduction setting, with special reference to the Harsit Pluton, Eastern Turkey

We present elemental and Sr-Nd-Pb isotopic data for the magmatic suite (similar to 79 Ma) of the Harsit pluton, from the Eastern Pontides (NE Turkey), with the aim of determining its magma source and geodynamic evolution. The pluton comprises granite, granodiorite, tonalite and minor diorite (SiO(2) = 59.43-76.95 wt%), with only minor gabbroic diorite mafic microgranular enclaves in composition (SiO(2) = 54.95-56.32 wt%), and exhibits low Mg# (<46). All samples show a high-K calc-alkaline differentiation trend and I-type features. The chondrite-normalized REE patterns are fractionated [(La/Yb)(n) = 2.40-12.44] and display weak Eu anomalies (Eu/Eu* = 0.30-0.76). The rocks are characterized by enrichment of LILE and depletion of HFSE. The Harsit host rocks have weak concave-upward REE patterns, suggesting that amphibole and garnet played a significant role in their generation during magma segregation. The host rocks and their enclaves are isotopically indistinguishable. Sr-Nd isotopic data for all of the samples display I(Sr) = 0.70676-0.70708, epsilon(Nd)(79 Ma) = -4.4 to -3.3, with T(DM) = 1.09-1.36 Ga. The lead isotopic ratios are ((206)Pb/(204)pb) = 18.79-18.87, ((207)Pb/(204)Pb) = 15.59-15.61 and ((208)Pb/(204)Pb) = 38.71-38.83. These geochemical data rule out pure crustal-derived magma genesis in a post-collision extensional stage and suggest mixed-origin magma generation in a subduction setting. The melting that generated these high-K granitoidic rocks may have resulted from the upper Cretaceous subduction of the Izmir-Ankara-Erzincan oceanic slab beneath the Eurasian block in the region. The back-arc extensional events would have caused melting of the enriched subcontinental lithospheric mantle and formed mafic magma. The underplating of the lower crust by mafic magmas would have played a significant role in the generation of high-K magma. Thus, a thermal anomaly induced by underplated basic magma into a hot crust would have caused partial melting in the lower part of the crust. In this scenario, the lithospheric mantle-derived basaltic melt first mixed with granitic magma of crustal origin at depth. Then, the melts, which subsequently underwent a fractional crystallization and crustal assimilation processes, could ascend to shallower crustal levels to generate a variety of rock types ranging from diorite to granite. Sr-Nd isotope modeling shows that the generation of these magmas involved similar to 65-75% of the lower crustal-derived melt and similar to 25-35% of subcontinental lithospheric mantle. Further, geochemical data and the Ar-Ar plateau age on hornblende, combined with regional studies, imply that the Harsit pluton formed in a subduction setting and that the back-arc extensional period started by least similar to 79 Ma in the Eastern Pontides.Geochemistry & GeophysicsMineralogySCI(E)33ARTICLE4467-48716

Extreme 230Th excesses in magnesian andesites from Baja California

International audienceFour Late Pleistocene magnesian andesites (locally called "bajaites") from Baja California have been dated by the unspiked K-Ar method and analyzed for major, trace elements and Sr, Nd, Pb, Th and U isotopes. They display very large 230Th excesses (50% to 120%) relative to 238U. They also have very high Ba and Sr contents and Sr/Y and La/Yb ratios, together with suprachondritic Nb/Ta ratios (22 to 26). From correlations between 230Th excess and major elements, trace element and isotopic ratios, we show that the unusual geochemical signature of these lavas can be ascribed to very low partial melting degrees of a mantle source containing residual rutile. This mantle source derived from interactions between the Baja California lithospheric mantle and older magnesian andesite melts carrying a sedimentary component, which were formed during the Late Miocene opening of the Baja California asthenospheric window. During the Quaternary, this metasomatized source experienced dehydration melting triggered by the hot thermal regime due to the uprise of the subslab Pacific asthenosphere beneath Baja California and the Gulf of California

Volcanic markers of the post-subduction evolution of Baja California and Sonora, Mexico : slab tearing versus lithospheric rupture of the Gulf of California

Abstract?The study of the geochemical compositions and K-Ar or Ar-Ar ages of ca. 350 Neogene and Quaternary lavas from Baja California, the Gulf of California and Sonora allows us to discuss the nature of their mantle or crustal sources, the conditions of their melting and the tectonic regime prevailing during their genesis and emplacement. Nine petrographic/geochemical groups are distinguished: ??regular' calc-alkaline lavas; adakites; magnesian andesites and related basalts and basaltic andesites; niobium-enriched basalts; alkali basalts and trachybasalts; oceanic (MORB-type) basalts; tholeiitic/transitional basalts and basaltic andesites; peralkaline rhyolites (comendites); and icelandites. We show that the spatial and temporal distribution of these lava types provides constraints on their sources and the geodynamic setting controlling their partial melting. Three successive stages are distinguished. Between 23 and 13 Ma, calc-alkaline lavas linked to the subduction of the Pacific-Farallon plate formed the Comondu´ and central coast of the Sonora volcanic arc. In the extensional domain of western Sonora, lithospheric mantle-derived tholeiitic to transitional basalts and basaltic andesites were emplaced within the southern extension of the Basin and Range province. The end of the Farallon subduction was marked by the emplacement of much more complex Middle to Late Miocene volcanic associations, between 13 and 7 Ma. Calc-alkaline activity became sporadic and was replaced by unusual post-subduction magma types including adakites, niobium-enriched basalts, magnesian andesites, comendites and icelandites. The spatial and temporal distribution of these lavas is consistent with the development of a slab tear, evolving into a 200-km-wide slab window sub-parallel to the trench, and extending from the Pacific coast of Baja California to coastal Sonora. Tholeiitic, transitional and alkali basalts of subslab origin ascended through this window, and adakites derived from the partial melting of its upper lip, relatively close to the trench. Calcalkaline lavas, magnesian andesites and niobium-enriched basalts formed from hydrous melting of the supraslab mantle triggered by the uprise of hot Pacific asthenosphere through the window. During the Plio-Quaternary, the ??no-slab' regime following the sinking of the old part of the Farallon plate within the deep mantle allowed the emplacement of alkali and tholeiitic/transitional basalts of deep asthenospheric origin in Baja California and Sonora. The lithospheric rupture connected with the opening of the Gulf of California generated a high thermal regime associated to asthenospheric uprise and emplaced Quaternary depleted MORB-type tholeiites. This thermal regime also induced partial melting of the thinned lithospheric mantle of the Gulf area, generating calcalkaline lavas as well as adakites derived from slivers of oceanic crust incorporated within this mantle

Eocene to Quaternary mafic-intermediate volcanism in San Luis Potosí, central Mexico: The transition from Farallon plate subduction to intra-plate continental magmatism

International audienceThe San Luis Potosí Volcanic Field (SLPVF) of central Mexico includes volcanic sequences of felsic, intermediate and basic compositions that were erupted as discrete episodes from the Eocene to the Pleistocene. Volcanism was dominated by widespread and voluminous rhyolitic ignimbrites of the mid-Tertiary Ignimbrite Flare-up. However, the complete volcanic history must consider basaltic and andesitic Eocene-Pleistocene volcanic successions that provide key evidence for understanding the geochemical evolution of the volcanism in the SLPVF during this time span. Five sequences are recognized according to their geochemical characteristics, each comprising a volcano-tectonic episode. The first episode comprises basaltic andesites and andesites erupted during three intervals, 45-42 Ma, 36-31 Ma, and 31-30 Ma. The oldest was derived from subduction magmatism, whereas the youngest has an intra-plate magmatic signature and this represents the transition from the end of a long lasting subduction regime of the Farallon plate to the initiation of intra-plate continental extension in the North American plate. The second episode, at 29.5-28 Ma, comprises a bimodal succession of high-silica rhyolites and alkaline basalts (hawaiites) that are interpreted as magmatism generated in an intra-plate continental extension regime during the Basin and Range faulting. The third episode, at 21 Ma, is characterized by trachybasalts and trachyandesites that represent mantle basaltic melts that were contaminated through assimilation of the lower crust during advanced stage of intra-plate extension that started at Oligocene. The fourth episode includes 12 Ma alkaline basalts and andesites that were erupted from fissures. These mantle derived magmas evolved to andesites by crustal anatexis and crystal fractionation within a continued, extensional, intra-plate regime. Lastly, the fifth episode comprises 5.0 to 0.6 Ma alkaline basalts (basanites) containing mantle xenoliths, that were erupted from maars and tuff cones, which are the youngest manifestations of mantle-derived intra-plate extensional events. Based upon this volcanic record, the last subduction manifestations of the extinct Farallon plate occurred at about 42 Ma, this was followed by a transition to intra-plate magmatism between 42 and 31 Ma, and an extensional, intra-plate tectonic setting from 31 Ma to almost Present