Deconvolving the pre-Himalayan Indian margin-Tales of crustal growth and destruction

The metamorphic core of the Himalaya is composed of Indian cratonic rocks with two distinct crustal affinities that are defined by radiogenic isotopic geochemistry and detrital zircon age spectra. One is derived predominantly from the Paleoproterozoic and Archean rocks of the Indian cratonic interior and is either represented as metamorphosed sedimentary rocks of the Lesser Himalayan Sequence(LHS) or as slices of the distal cratonic margin. The other is the Greater Himalayan Sequence(GHS) whose provenance is less clear and has an enigmatic affinity. Here we present new detrital zircon Hf analyses from LHS and GHS samples spanning over 1000 km along the orogen that respectively show a striking similarity in age spectra and Hf isotope ratios. Within the GHS, the zircon age populations at 2800-2500 Ma,1800 Ma, 1000 Ma and 500 Ma can be ascribed to various Gondwanan source regions; however, a pervasive and dominant Tonianage population(～860-800 Ma) with a variably enriched radiogenic Hf isotope signature(eHf = 10 to-20) has not been identified from Gondwana or peripheral accreted terranes. We suggest this detrital zircon age population was derived from a crustal province that was subsequently removed by tectonic erosion. Substantial geologic evidence exists from previous studies across the Himalaya supporting the Cambro-Ordovician Kurgiakh Orogeny. We propose the tectonic removal of Tonian lithosphere occurred prior to or during this Cambro-Ordovician episode of orogenesis in a similar scenario as is seen in the modern Andean and Indonesian orogenies, wherein tectonic processes have removed significant portions of the continental lithosphere in a relatively short amount of time. This model described herein of the pre-Himalayan northern margin of Greater India highlights the paucity of the geologic record associated with the growth of continental crust. Although the continental crust is the archive of Earth history, it is vital to recognize the ways in which preservation bias and destruction of continental crust informs geologic models.

A Laurentian affinity for the Embu Terrane,Ribeira Belt(SE Brazil),revealed by zircon provenance statistical analysis

New and compiled detrital zircon U-Pb ages from the southern Neoproterozoic-Cambrian Ribeira Belt,SE Brazil,demonstrate Laurentian affinity of the Embu Terrane which is statistically distinct from the adjoining Apiaíand São Roque terranes with cratonic affinity(e.g.,São Francisco Craton).Zircon provenance results indicate that the type-area of the Embu Terrane is dominated by detrital zircon age modes at ca.1200 Ma,1400 Ma,and 1800 Ma,with maximum depositional age of ca.1000 Ma.In contrast,the Apiaíand São Roque terranes are dominated by Paleoproterozoic detrital zircon ages(ca.2200-2000 Ma age dominant component),with maximum depositional ages of ca.1400 Ma and 1750 Ma,respectively.Multidimensional scaling(MDS)analysis of non-parametric similarity measurements on zircon age populations indicates for the first time that the Embu Terrane encompass two statistically distinct detrital zircon age spectra,which is also reflected in the metamorphic zircon age record.The statistical characterization of the Embu Terrane through populational metrics allow a quantitative comparison with surrounding tectonic domains and rock samples classified such as Embu-type.Our results clearly highlight the distinction between the statistically differentiated Embu Terrane from the Apiaíand São Roque terranes,supporting an allochthonous interpretation.In addition,we demonstrate that rocks samples previously classified as Embu-type are significantly dissimilar to the definition of Embu Terrane,failing to support alternative tectonic models(e.g.,intracontinental evolution).Detrital zircon age spectra reveal that the Apiaíand São Roque terranes have similar zircon provenance to domains sourced from the São Francisco Craton,whereas detrital zircon populations from the Embu Terrane have greater affinity with SW Laurentia basins(and their inferred sediment sources),consistent with previous findings.Therefore,we interpret the Embu Terrane as a Rodinia descendant developed along the active margin of the SW Laurentia that collided with the Ribeira Belt during early Neoproterozoic(810-760 Ma).