Combined Geochemical Approach Research Articles

Geochemical analysis of Tequesta calcretes

Members of the Tequesta, a pre/proto-historic indigenous group of South Florida, cut thousands of postholes into the shallow limestone bedrock of the Metropolitan Square site, located in present-day Miami. Calcrete (limestone soil crust) formations found in and around these postholes serve as a geochemical repository attesting to various human and natural processes at the site. We present here the results of petrographic, total organic carbon (TOC), X-ray diffraction (XRD), X-ray fluorescence (XRF), and stable-isotope analysis, which were used in an attempt to relatively date posthole excavation and to identify patterns of changing site use. Ultimately, the combination of these methods yielded useful insights into paleoenvironment and proposed changes in site use, while failing to provide desired information on chronology. A combined geochemical approach such as that employed here would seem to hold potential for future study of sites located in calcareous/karstic contexts.

Post-collisional transition from calc-alkaline to alkaline volcanism during the Neogene in Oranie (Algeria): magmatic expression of a slab breakoff

During the Neogene, a magmatic change from calc-alkaline to alkaline types occurred in all the regions surrounding the western Mediterranean. This change has been studied in Oranie (western Algeria). In this area, potassic to shoshonitic calc-alkaline andesites (with La/Nb ratios in the range 4–6) were mainly erupted between 12 and 9 Ma. They were followed (between 10 and 7 Ma) by basalts displaying geochemical features which are transitional between calc-alkaline and alkaline lavas (La/Nb=1–1.7). After a ca. 3-Ma quiescence period, volcanic activity resumed, with the eruption of OIB-type alkaline basalts (La/Nb=0.5–0.6), from 4 to 0.8 Ma. A combined geochemical approach, using incompatible elements and Sr, Nd and O isotopes, allows us to conclude that the transitional basalts derived from the melting of a heterogeneous mantle source, at the boundary between lithosphere and asthenosphere. We propose that melting of a previously subduction-modified lithospheric mantle occurred between 12 and 10 Ma, in response to the upwelling of hot asthenosphere flowing up into an opening gap above a detached sinking slab. As a result, calc-alkaline magmas were formed. From 10 to 7 Ma, the transitional basalts were generated through melting of the boundary mantle zone between the lithosphere and the upwelling asthenosphere. During that stage, the contribution of the lithospheric source was still predominant. Then, as sinking of the oceanic slab progressed, the increasing uprise of the asthenosphere led to the formation and emplacement (from 4 to 0.8 Ma) of typical within-plate alkaline basalts derived from a plume-modified asthenospheric mantle.