Global volcanic arc magma composition correlates with thickness of both arc crust(Moho depth)and arc lithosphere(LAB depth):A revealing message on arc basement histories and arc magmatism

Past fifty years have seen mounting publications on the genesis of volcanic arc magmas.While details remain debated,it is generally agreed that arc magmas result from slab-dehydration induced mantle wedge melting followed by crustal level differentiation of varying extent and sophistication.Two recent arc magma studies deserve particular attention because they attempt to discuss globally unifying controls on arc magma composition.Both Harvard study(Turner and Langmuir,2015a,b)and Rice study(Farner and Lee,2017)show correlations of arc magma composition with crustal thickness and both ascribe the crustal thickness as the principal control on their observed magma compositional variations,yet the physical role of the crustal thickness in their interpretations is markedly different because of(1)the ambiguous use of“crust”and(2)their different magma compositional ranges chosen in discussion.The Harvard study only uses basaltic samples corrected to MgO=6.0 wt.%to discuss mantle processes and interprets the arc crustal thickness as restricting the mantle wedge melting,i.e.,the extent of melting decreases with increasing crustal thickness.The Rice study uses samples of all compositions(basaltic to rhyolitic),whose extent of differentiation increases with increasing crustal thickness,interpreted as Moho-crossing mantle wedge melts travelling greater vertical distance with greater degree of cooling and erupting more evolved compositions above thicker crust than melts erupted above thinner crust without need of invoking mantle wedge processes.We commend these efforts and approve their different approaches but emphasize that the unifying understanding of global arc magmatism requires clearly defined Moho(the base of the crust)and LAB(the lithosphere-asthenosphere boundary)and their intrinsic controls on mantle wedge melting(Harvard Study model)and crustal level magma differentiation(Rice Study model)beneath global arcs.In this study,we use chemical compositions of 36,945 global arc volcanic samples provided by the Rice study together with the literature data on seismic Moho and LAB depths of these sample locations to establish(1)the correlation of crustal thickness interval averaged magma composition with crustal thickness and(2)the correlation of lithosphere thickness interval averaged magma composition with lithosphere thickness.These correlations reaffirm our understanding that the lithospheric mantle must exist beneath volcanic arc crust with a globally averaged LAB/Moho depth ratio of 3.26±0.63,i.e.,the arc crust is on average about 31.8%±6.1%of the lithosphere thickness.This knowledge forms a solid constraint essential for models of global arc magmatism.

Late Jurassic Volcanism Deduced from Geochemical,Geochronological,and Sr-Nd-Hf Isotopic Composition Characteristics of the Nanyuan Formation,South China

The Nanyuan Formation contains information related to the Mesozoic tectonic transformation.In this study,three representative profiles were surveyed from the Nanyuan Formation,and multiple analyses were conducted.Zircon U-Pb dating yielded their ages as approximately 158–146 Ma.The volcanic rocks are enriched in Rb,Th,U,K,and Pb and depleted in Nb,Ta,P,and Ti,implying their affinity for I-type granites.TheεNd(t)values(-8.3 to-6.0),^(87)Sr/^(86)Sr)i values(0.7077–0.7094)of the volcanic rock,andεHf(t)values(-8.71 to 0.12)of the Mesozoic zircons suggest that the Nanyuan Formation magma originated in the lower crust with the involvement of depleted mantle materials.The parent rocks of the rhyolitic and dacitic volcanic rocks formed by partial melting of basement rocks in South China and the andesitic volcanic rocks were derived from partial melting of the metasomatites generated by slab-mantle interaction.The fractional crystallization also played an important role in later stage.Discrimination diagrams of the volcanic rocks indicated that they formed in a volcanic arc environment.Combined with previous data,the Nanyuan Formation recorded subduction of the Paleo-Pacific Plate before regional tectonic transformation.The compressive stress field endured until the end of the Late Jurassic.

Cretaceous tectonic evolution of the Neo-Tethys in Central Iran:Evidence from petrology and age of the Nain-Ashin ophiolitic basalts

The Nain and Ashin ophiolites consist of Mesozoic melange units that were emplaced in the Late Cretaceous onto the continental basement of the Central-East Iran microcontinent(CEIM).They largely consist of serpentinized peridotites slices;nonetheless,minor tectonic slices of sheeted dykes and pillow lavas-locally stratigraphically associated with radiolarian cherts-can be found in these ophiolitic melanges.Based on their whole rock geochemistry and mineral chemistry,these rocks can be divided into two geochemical groups.The sheeted dykes and most of the pillow lavas show island arc tholeiitic(IAT)affinity,whereas a few pillow lavas from the Nain ophiolites show calc-alkaline(CA)affinity.Petrogenetic modeling based on trace elements composition indicates that both IAT and CA rocks derived from partial melting of depleted mantle sources that underwent enrichment in subduction-derived components prior to melting.Petrogenetic modeling shows that these components were represented by pure aqueous fluids,or sediment melts,or a combination of both,suggesting that the studied rocks were formed in an arc-forearc tectonic setting.Our new biostratigraphic data indicate this arc-forearc setting was active in the Early Cretaceous.Previous tectonic interpretations suggested that the Nain ophiolites formed,in a Late Cretaceous backarc basin located in the south of the CEIM(the so-called Nain-Baft basin).However,recent studies showed that the CEIM underwent a counter-clockwise rotation in the Cenozoic,which displaced the Nain and Ashin ophiolites in their present day position from an original northeastward location.This evidence combined with our new data and a comparison of the chemical features of volcanic rocks from different ophiolites around the CEIM allow us to suggest that the Nain-Ashin volcanic rocks and dykes were formed in a volcanic arc that developed on the northern margin of the CEIM during the Early Cretaceous in association with the subduction,below the CEIM,of a Neo-Tethys oceanic branch that was existing between the CEIM and the southern margin of Eurasia.As a major conclusion of this paper,a new geodynamic model for the Cretaceous evolution of the CEIM and surrounding Neo-Tethyan oceanic basins is proposed.

Comparative Geochemical and Petrographic Studies of the Various Granitoids between Central and Western Arm, in the Central Part of Ramagiri Schist Belt and Their Petrogenetic Histories

Granitoids between the central and western arm of Ramagiri schist belt in its central part, are broadly classified into the migmatite gneiss, grey granodiorite and pink monzogranite, based on field characteristics and petrographic features. These granitoids belong to the Tonalite-Granodiorite-Monzogranite (TGM) suite of PGC-II. All the samples are fresh as per the CIA values, PC1-PC2 binary plot and MFW ternary plot. The granodiorites occupy the expected field in the normative IUGS, TAS, and R1-R2 classification diagrams, but the monzogranites occupy the monzogranite field in the normative IUGS classification diagram and granite to alkali granite field in the rest. The granodiorites exhibit both ferroan to magnesian, alkali-calcic nature with metaluminous I type features and falls in the calc-alkaline to high K calc-alkaline series. They have high ΣREE (an average 327.905 ppm) content, and show LREE enrichment ((La/Sm)N = 3.1 - 6.8) with enriched but relatively flat HREE ((Gd/Yb)N = 1.75 - 5.26) patterns and weak negative to positive Eu anomaly (Eu/Eu* = 0.62 - 1.18). The monzogranites, on the other hand, are peraluminous, alkalic, ferroan, high K calc-alkaline, S-type granites, exhibiting relatively low ΣREE (an average 118.693 ppm) contents, strongly fractionated REE patterns with highly enriched LREE ((La/Sm)N =1.74 - 9.76), depleted HREE ((Gd/Yb)N = 0.43 - 2.21) patterns having concave upward shape, and strong negative Eu anomaly (Eu/Eu* = 0.23 - 0.89). Geothermobarometry revealed the average emplacement temperature and pressure of the granodiorites and monzogranites as 812.5℃, 8.14 ± 0.6 kbar and 775℃, 3.14 kbar, respectively. Based, on the observations, it can be concluded that the granodiorites have formed in volcanic arc setting by partial melting of the lower crust and S-type monzogranites have been produced at a relatively shallower depth in the crust, by continental crust recycling due to hydrothermal influx.