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The Xiadong mafic-ultramafic complex lies in the central part of the Middle Tianshan Massif (MTM), along the southern margin of the Central Asian Orogenic Belt (CAOB). This complex is composed of dunite, hornblende (Hbl) clinopyroxenite, hornblendite, and Hbl gabbro. These rocks are characterized by adcumulated textures and variable alteration. Orthopyroxene is an extremely rare mineral in all rock units and plagioclase is absent in dunite and Hbl clinopyroxenite. Hbl, Fe-chromite, and Cr-magnetite are common phases. Olivines have forsterite (Fo) contents ranging from 92.3 to 96.6. Clinopyroxenes are Ca-rich, Ti-poor diopsides, and mostly altered to tremolites or actinolites. Chromites display low TiO 2 and Al 2 O 3 contents and high Cr# and Fe 2+ /(Fe 2+ + Mg) values. Primary and secondary Hbls show wide compositional variations. These petrological and mineralogical features as well as mineral chemistry are comparable to typical Alaskan-type complexes worldwide, which are widely considered to have formed above subduction zones. The chemistry of clinopyroxene and chromite supports an arc plate-tectonic origin for the Xiadong complex. Its confirmation as an Alaskan-type complex implies that the MTM, with Precambrian basement, was probably a continental arc during oceanic plate underflow and further supports the hypothesis of southward subduction of the Palaeozoic Junggar Ocean.

Metagraywackes and metapelites from the Paleoproterozoic Birimian Supergroup in the Birim diamondiferous field, southern Ghana, were analyzed for their major and trace element contents. Compared to early Proterozoic crust, the metasedimentary rocks are enriched in ferromagnesian elements but depleted in rare earth elements (REE), high field strength elements (HFSE) (with exception of Zr), and Th. They show REE patterns similar to their Archean counterparts. The chemical data indicate that the sediments were derived from a local source of mixed felsic-mafic composition, with the latter dominating. The source rocks were the basaltic to dacitic volcanic rocks and granitoids within the Birimian greenstone belts. The chemical data further suggest their deposition in a tectonic setting comparable to modern island arcs, and that minimal old upper crust (i.e., pre-Birimian sources) was involved in their formation. The analyzed metasedimentary rocks have Eu-anomalies and Gd N /Yb N , Sm/Nd, Th/Sc, Cr/Sc and Cr/Th ratios that closely resemble those of their Archean counterparts, and therefore inconsistent with models suggesting abrupt compositional changes in upper crust at the Archean-Proterozoic boundary.

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