Search

Abstract: We present new data on the geologic position, composition, and isotope characteristics of the Early Cretaceous granitoids of the Samarka terrane, Sikhote-Alin’, formed on a transform continental margin. Geological and geochronological data show that these granitoids were generated at two stages of magmatism: in the first half (Hauterivian–Barremian, 130–123 Ma) and second half (Albian–Cenomanian, 110–98 Ma) of the Early Cretaceous. Granitoids of the first stage form an autonomous (free of basic precursors) unimodal melanogranite–granite association and are characterized by normal alkalinity with domination of K over Na, low contents of Ca, and elevated contents of Al2O3. By composition, these are S-granites with a model Nd age of ∼1.3 Ga. Granitoids of the second stage are of more diverse petrogeochemical types. They show wider variations in K/Na and A/CNK, are richer in Ca and, sometimes, Sr, and are poorer in P than the granitoids of the first stage. Their compositions form a continuous trend from S- to I-granites, and their model Nd age is ≤1.2 Ga. Comparison of the petrochemical, trace-element, and isotope characteristics of the Early Cretaceous granitoids and upper-crustal rocks (sandstones and siltstones of the turbidite matrix of a Jurassic accretionary prism and basalts from the inclusions in it) of the Samarka terrane and the coeval garrboids has shown that the potassic S-granitoids formed at the early (Hauterivian–Barremian) stage of magmatism as a result of the anatexis of upper-crustal sedimentary rocks. At the late (Albian–Early Cenomanian) stage, the intrusion of mantle magmas led to a temperature increase in the lower crust, which favored more active anatexis, involvement of high-melting substrates (oceanic basalts) in the granite formation, and interaction of mantle and crustal magmas. This resulted in a great diversity of granitoids (from S- to I-type). [Copyright &y& Elsevier]

Abstract: We generalize results of geological, geochronological, geochemical, and isotope-geochemical studies of the Vendian–Early Cambrian island-arc plagiogranitoid magmatism in the Altai–Sayan folded area and in the Lake Zone of western Mongolia. Based on these data, we analyzed the scales of development of plagiogranitoid magmatism, studied the petrologic composition and isotope characteristics of granitoids, and established the main sources of plagiogranitoid-generating melts and the leading mechanisms of formation of Early Caledonian juvenile crust. [Copyright &y& Elsevier]

[Display omitted] • The Doros dolerites and East Anabar dolerites have ages of 1.76 and 1.78 Ga, respectively. • There is a 20 my difference between late Paleoproterozoic mafic magmatism in the north and south Siberia. • 1.78–1.75 Ga mafic dykes and intrusions of Siberia were generated mainly from the subcontinental lithospheric mantle. • For this period there is continuous mafic magmatism in some continents and short magmatic events in other continents. • 1.79–1.75 Ga mafic intrusions around the world demonstrate the prevalence of subcontinental lithospheric mantle source. The paper presents new geological, geochronological, geochemical, and Nd isotopic data on the late Paleoproterozoic dolerites of the Siberian craton. U-Pb baddeleyite ages of the Doros dolerites (Aldan shield, south-eastern Siberia) and East Anabar dolerites (Anabar shield, northern Siberia) are 1757 ± 4 and 1774 ± 6 Ma, respectively. This new geochronological data together with published 1.76–1.75 Ga ages for Timpton-Algamay dolerites of the Aldan shield and Chaya dolerites of the Baikal uplift (southern Siberia) revealed a 20 my difference between this magmatism in the north and south Siberia. The Doros dolerites in their chemical compositions correspond to sub-alkaline basaltic andesites, but the East Anabar dolerite is chemically close to basaltic trachyandesite. The Doros dolerites demonstrate negative and close to zero ɛNd(t) values (from −7.0 to +0.1), which correlate well with SiO 2 and Mg#. All Doros dolerites have Nb–Ta and Ti anomalies in multielement spectra. The geochemical and Nd isotopic data suggest that the Doros dolerites have been formed by the mixing of mantle and crustal material. The East Anabar dolerites are characterized by a negative ɛNd(t) value of −3.7, negative Nb–Ta anomaly in multielement spectra, high concentrations of TiO 2 and P 2 O 5. The 1775 Ma East Anabar dolerites could be generated from a subcontinental lithospheric mantle source, possibly with some mantle plume interaction (possibly centred at the convergence with the 1.76–1.75 Ga fanning Aldan swarm). Geochemistry and Nd isotope systematics of all 1.78–1.75 Ga mafic dykes and intrusions of the Siberian craton indicate the subcontinental lithospheric mantle source or mantle source contaminated by crustal material. Geochronological data from the 1.79–1.75 Ga magmatic rocks of Siberia and other continents suggest continuous magmatism over this interval in some continents, but a series of short magmatic events/pulses (from one to four) separated by intervals of quiescence in other continents. We locate the analysed dykes and sills on new 1750 Ma and 1790 Ma global paleogeographic reconstructions. Analysis of 1.79–1.75 Ga geochemical data on mafic intrusions from Late Paleoproterozoic continents suggests the prevalence of subcontinental lithospheric mantle source for the mafic intrusions over the pure mantle plume source. [ABSTRACT FROM AUTHOR]