Stable oxygen and carbon isotope compositions of Campanian grainstones and rudstones

Petrographic descriptions and stable oxygen and carbon isotope compositions of microsamples of Campanian-age sediment gravity-flow deposits from Northeast Providence Channel, Bahamas, indicate deep-marine cementation of shallow-marine skeletal grains that were transported to the channel during the Late Cretaceous. Shallow-marine components are represented by mollusks, especially rudists, and shallow-water benthic foraminifers as well as sparse echinoderm and algal grains. The sole evidence of diagenesis in shallow-marine environments consists of micrite envelopes around skeletal grains. Shallow-marine skeletal grains have mean stable isotope values of -3.1 per mil d18O and +2.6 per mil d13C. The d18O values are consistent with precipitation in equilibrium with warm (20°-30°C), shallow-marine water. Deep-marine components are represented by equant calcite spar cements and rip-up clasts of slope sediments. Spar cements, exhibiting hexagonal morphology with scalenohedral terminations, most commonly occur as thin isopachous linings in the abundant porosity. Deep-marine cements have mean stable isotope values of - 1.1 per mil d18O and +2.7 per mil d13C. Deep-marine cements are 18O-enriched relative to shallow-marine skeletal grains, consistent with precipitation in equilibrium with colder (10°-20°C), deep-marine waters. The cement .source during lithification appears to have been dissolution of aragonite and high-magnesium calcite skeletal grains, which made up part of the transported sediment. Interbedded periplatform ooze remains uncemented, or poorly cemented, probably because of lower permeability. Equant spar cements that occur in gravity-flow deposits recovered from Hole 634A have stable isotope compositions similar to spars in Lower and mid-Cretaceous shallow-water limestones exposed on the Bahama Escarpment, to Campanian-Paleocene deep-marine hardgrounds recovered during DSDP Leg 15 in the Caribbean, and to spars in Aptian-Albian talus deposits at the base of the Campeche Escarpment recovered during DSDP Leg 77.

Major and trace element compositions of the Buendnerschiefer samples

The Bündnerschiefer of the Swiss-Italian Alps is a large sedimentary complex deposited on the Piemonte-Liguria and Valais oceans and associated continental margins from the upper Jurassic to Eocene. It is made of a large variety of sequences associated or not with an ophiolitic basement. The Bündnerschiefer makes an accretionary prism that developed syn-tectonically from the onset of alpine subduction, and it records orogenic metamorphism following episodes of HP metamorphism. The Bündnerschiefer shares important similarities with the Otago schists of New Zealand and with the Wepawaug schists of Connecticut, both of which form accretionary prisms and have an orogenic metamorphic imprint. With the aim of testing the hypothesis of mobility of chemical components as a function of metamorphic grade, in this work I present fifty-five bulk chemical analyses of various lithological facies of the Bündnerschiefer collected along the well-studied field gradient of the Lepontine dome of Central Switzerland, in the Prättigau half window of East Switzerland, and in the Tsaté Nappe of Valle d'Aosta (Italy). The dataset includes the concentration of major components, large ion lithophile elements (Rb, Sr, Ba, Cs), high field strength elements (Zr, Ti, Nb, Th, U, Ta, Hf), fluid-mobile light elements (B, Li), volatiles (CO2, S), REEs, and Y, V, Cr, Co, Sn, Pb, Cu, Zn, Tl, Sb, Be, and Au. These data are compared against the compositions of the global marine sediment reservoir, typical crustal reservoirs, and against the previously measured compositions of Otago and Wepawaug schists. Results reveal that, irrespective of their metamorphic evolution, the bulk chemical compositions of orogenic metasediments are characterized by mostly constant compositional ratios (e.g., K2O/Al2O3, Ba/Al2O3, Sr/CaO, etc.), whose values in most cases are undistinguishable from those of actual marine sediments and other crustal reservoirs. For these rocks, only volatile concentrations decrease dramatically as a function of metamorphic temperature, and significant deviations from the reservoir signatures are evident for SiO2, B, and Li. These results are interpreted as an indication of residual enrichment in the sediments, a process taking place during syn-metamorphic dehydration from the onset of metamorphism in a regime of chemical immobility. Residual enrichment increased the absolute concentrations of the chemical components of these rocks, but did not modify significantly their fundamental ratios. This poor compositional modification of the sediments indicates that orogenic metamorphism in general does not promote significant mass transfer from accretionary prisms. In contrast, mass transfer calculations carried out in a shear zone crosscutting the Bündnerschiefer shows that significant mass transfer occurs within these narrow zones, resulting in gains of H2O, SiO2, Al2O3, K2O, Ba, Y, Rb, Cu, V, Tl, Mo, and Ce during deformation and loss of Na2O, CO2, S, Ni, B, U, and Pb from the rock. These components were presumably transported by an aquo-carbonic fluid along the shear zone. These distinct attitudes to mobilize chemical elements from orogenic sediments may have implications for a potentially large number of geochemical processes in active continental margins, from the recycling of chemical components at plate margins to the genesis of hydrothermal ore deposits.