Petrology and geochronology of ultrahigh-pressure granitic gneiss from South Dulan, North Qaidam belt, NW China

<div><p>abstract</p><p>An integrated study including petrography, mineral chemistry, metamorphic P–T path modelling, and zircon U–Pb dating was conducted on a granitic gneiss and enclosed eclogite from South Dulan, North Qaidam UHP (ultrahigh-pressure) belt. The result shows that the granitic gneiss underwent a clockwise P–T path with a peak-P stage at 655–745°C, 30–34 kbar, and a subsequent peak-T stage at 815–870°C, 14–18 kbar, which is similar to the P–T estimates reported for coesite-bearing continental-type eclogites in this region. The enclosed eclogite resembles an olivine–pyroxene-rich cumulate in Qaidam block. It has a similar prograde P–T path with the country gneiss and experienced a peak-P stage of 682–748°C at 27–34 kbar. Zircon U–Pb dating yields an eclogite-facies metamorphic age of 447 ± 2 Ma for the granitic gneiss and 445 ± 6 Ma for the enclosed eclogite. These ages agree with metamorphic ages obtained from paragneisses (427–439 Ma), coesite-bearing continental-type eclogites (430–451 Ma), and UHPM (ultrahigh-pressure metamorphic) oceanic crust–mantle sequence (440–445 Ma) from South Dulan, as well as UHP eclogites, garnet peridotite, and gneisses from other units (460–420 Ma) within this belt reported by others. Similar metamorphic ages as well as P–T evolution documented in gneisses and intercalated eclogites imply that both rocks experienced a coeval UHP event. Summarizing all the published geochronology data, we argue that the North Qaidam UHP belt was mainly formed by continental deep subduction at ~460 to ~420 Ma. The UHPM oceanic crust-mantle sequence in South Dulan may represent oceanic lithosphere in the transition zone between oceanic and continental crust, which was dragged upward by the exhumed continental rocks after break-off of the dense oceanic crust.</p></div

Rubidium (Rb) versus germanium (Ge) concentration plot of the Danian geological samples and the LBF material.

<p>Rubidium (Rb) versus germanium (Ge) concentration plot of the Danian geological samples and the LBF material.</p

A multi-technique analytical approach to sourcing Scandinavian flint: Provenance of ballast flint from the shipwreck “Leirvigen 1”, Norway

<div><p>Although Scandinavian flint is one of the most important materials used for prehistoric stone tool production in Northern and Central Europe, a conclusive method for securely differentiating between flint sources, geologically bound to northern European chalk formations, has never been achieved. The main problems with traditional approaches concern the oftentimes high similarities of SiO₂ raw materials (i.e. chert and flint) on different scales due to similar genetic conditions and higher intra- than inter-source variation. Conventional chert and flint provenance studies chiefly concentrate on visual, petrographic or geochemical investigations. Hence, attempts to generate characteristic fingerprints of particular chert raw materials were in most cases unsatisfying. Here we show that the Multi Layered Chert Sourcing Approach (MLA) achieves a clear differentiation between primary sources of Scandinavian flint. The MLA combines visual comparative studies, stereo-microscopic analyses of microfossil inclusions, geochemical trace element analyses applying LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) and statistical analyses through CODA (Compositional Data Analysis). For archaeologists, provenance studies are the gateway to advance interpretations of economic behavior expressed in resource management strategies entailing the procurement, use and distribution of lithic raw materials. We demonstrate the relevance of our results for archaeological materials in a case study in which we were able to differentiate between Scandinavian flint sources and establish the provenance of historic ballast flint from a shipwreck found near Kristiansand close to the shore of southern Norway from a beach source in Northern Jutland, the Vigsø Bay.</p></div

Aluminum (Al) versus germanium (Ge) concentration plot of all Maastrichtian geological samples.

<p>Aluminum (Al) versus germanium (Ge) concentration plot of all Maastrichtian geological samples.</p

Linear discriminant plot of all Danian geological samples and the LBF material.

<p>Linear discriminant plot of all Danian geological samples and the LBF material.</p

Strontium (Sr) versus aluminum (Al) concentration plot of all geological samples.

<p>Strontium (Sr) versus aluminum (Al) concentration plot of all geological samples.</p

Strontium (Sr) versus magnesium (Mg) concentration plot of all geological samples.

<p>Strontium (Sr) versus magnesium (Mg) concentration plot of all geological samples.</p

Overview over the Vigsø Bay in Northern Jutland, Denmark.

<p>Overview over the Vigsø Bay in Northern Jutland, Denmark.</p

Strontium (Sr) versus aluminum (Al) concentration plot of all Maastrichtian geological samples.

<p>Strontium (Sr) versus aluminum (Al) concentration plot of all Maastrichtian geological samples.</p

Aluminum (Al) versus magnesium (Mg) concentration plot of all geological samples.

<p>Aluminum (Al) versus magnesium (Mg) concentration plot of all geological samples.</p