Abstract The Bear Valley Intrusive Suite (BVIS), located in the southernmost Sierra Nevada Batholith (SNB; California, USA) exposes a transcrustal magma system consisting of lower-crustal gabbros and volumetrically extensive middle- and upper-crustal tonalites. New chemical abrasion–isotope dilution–thermal ionization mass spectrometry U-Pb geochronology shows that the bulk of this ca. 100 Ma magmatic system crystallized in 1.39 ± 0.06 m.y. and was constructed with ultrahigh magmatic fluxes (∼250 km3/km/m.y.). This magmatic flux is roughly a factor of three greater than estimates for the SNB-wide flux during the Late Cretaceous flare-up, showing that individual magmatic systems can be constructed at extremely rapid rates. Further, the Hf isotopic composition of the BVIS (εHfi ∼–2 to +4) only allows for limited (∼25%) crustal assimilation. Our results show that the high magmatic fluxes recorded in the BVIS were dominantly derived from the mantle, and that “flare-up”–like local magmatic fluxes can be produced without extraordinary assimilation of crustal material.
The origin of pyroxenites and their relation to melt migration in the mantle have been investigated in two pyroxenite-rich zones in the Beni Bousera massif. Based on combined field, microtextural, mineralogical and geochemical observations, the pyroxenites were separated into four types. Type-I Cr-diopside websterites contain bright green diopside and have primitive bulk Ni, Cr and Mg-number. Their trace element systematics are characterized by slight light rare earth element (LREE) enrichment compared with the middle (MREE) and heavy (H)REE, and negative high field strength element (HFSE) anomalies in bulk-rock and mineral compositions suggesting that they result from melting of metasomatized mantle. Trace element concentrations of melts calculated to be in equilibrium with Type-I cpx have a subduction-like signature and show a close similarity to certain lavas erupted in the Alboran Basin. Calculated mineral equilibration temperatures of ∼1200 to 1350°C are close to the basalt liquidus and higher than for other pyroxenite types in Beni Bousera, which generally yield <1100°C. Type-II spinel websterites are also primitive, but contain augitic clinopyroxene; their whole-rock compositions are characterized by high Ti, Ni, and Mg-number, intermediate Cr and trace element patterns with LREE depletion over the MREE and HREE. Type-III garnet pyroxenites, which include the famous diamond-pseudomorph-bearing garnet pyroxenites, are more evolved than Types-I and -II and have low and variable Mg-number correlating with an Fe-enrichment trend. High bulk-rock and garnet HREE to LREE ratios result from high-pressure fractionation of garnet and augitic cpx at calculated pressures of >45 to 20–30 kbar. Type-III pyroxenites display strong variations of LREE and HFSE depletion and strong bulk Nb/Ta fractionation. Calculated melts in equilibrium with augitic cpx are variably enriched in incompatible trace elements similar to intraplate basalts. Type-IV pyroxenites are composed of green diopside, opx, garnet and plagioclase and/or spinel. Whole-rocks have high Na2O, CaO and Al2O3 concentrations and high Mg-number, are HREE depleted, and have positive Eu and Sr anomalies. Garnets are characterized by low HREE/MREE and positive Eu anomalies. The absence of bulk-rock HREE enrichment indicates a metamorphic origin for this garnet, which is corroborated by the presence of Al-rich metamorphic spinels. Relict magmatic plagioclase indicates a shallower (<10 kbar) crustal origin for these pyroxenites. Their metamorphic assemblage yields temperatures and pressures of 800–980°C and 14 kbar, indicating a pressure increase during the metamorphic overprint. The whole-rock geochemistry of Type-IV pyroxenites is comparable with that of rocks from the lower crustal section of the Kohistan (northern Pakistan) paleo-arc, indicating a possible origin of these rocks as cumulates in the deeper arc crust and subsequent delamination into the underlying mantle.
Significance We present paleomagnetic constraints on the latitude of an intraoceanic subduction system that is now sutured between India and Eurasia in the western Himalaya. Our results demonstrate that the India–Eurasia collision was a multistage process involving at least two subduction systems rather than a single-stage event. This resolves the discrepancy between the amount of convergence and the observed crustal shortening in the India–Eurasia collision system, as well as the 10–15 Ma time lag between collision onset in India and the initiation of collision-related deformation and metamorphism in Eurasia. The presence of an additional subduction system in the Neotethys ocean explains the rapid India–Eurasia convergence rates in the Cretaceous and global climate variations in the Cenozoic.
