Article

Eruptive Rocks

Soil Science (Impact Factor: 1.05). 06/1948; 66(1):78.
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    ABSTRACT: The Wyoming province, a small, ca. 500 000 km2 Archean craton, is the most southwestern of the Archean provinces in North America. It is composed primarily of Late Archean potassium-rich granitic rocks. In contrast to many other Archean provinces, rocks of tonalite-trondhjemite affinity are rare over most of the province and are restricted to rocks older than 2.8 Ga. Field, petrologic, geochemical and isotopic study of the Late Archean granites exposed in the Wind River Range have allowed us to identify at least four periods of potassic calc-alkalic magmatism at ∼2.8, 2.67, 2.63 and 2.55 Ga. Granitic rocks of these ages appear to be widespread across the Wyoming province. The oldest calc-alkalic granites of the Wind River Range, emplaced at ca. 2.8 Ga, appear to be derived predominantly from pre-existing crust. However, Nd isotopic data suggest that these granites cannot be the product solely of partial melting of older tonalitic gray gneisses. During at least two other periods of plutonism, at 2.67 and 2.63 Ga, generation of the Wind River Range batholiths involved the incorporation of substantial amounts of isotopically juvenile material, either from depleted mantle or young continental crust. The information presented below, as well as data available from elsewhere in the Wyoming province, is interpreted to suggest that the Wyoming province, unlike other Archean cratons, is not composed of a tectonic amalgamation of smaller, exotic terranes. Although the Wyoming province did experience crustal addition in Archean time, it was not by lateral accretion, but by incorporation of mantle-derived melts into large granitic batholiths.
    Precambrian Research 06/1998; 89(3). · 4.44 Impact Factor
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    ABSTRACT: The Central Anatolia Crystalline Complex (CACC) is characterized by Late Cretaceous high-temperature metamorphic rocks intruded by S-, I-, and A-type granitoids. Coeval basic plutonic and volcanic rocks also crop out in the complex. The NE–SW-trending Karacaali Magmatic Complex (KMC) represents a clear example of synchronous basic and acidic magmatic associations. We present new data on this coeval magmatism. The KMC plutonic rocks mainly consist of monzonite, granite, and gabbro, whereas the associated volcanic rocks are chiefly of basalt and rhyolite. All of the units have been cut by quartz, quartz-tourmaline, and calcite veins and by porphyritic leucogranite, aplitic, and basaltic dikes. The rhyolitic, basaltic, and gabbroic samples yield well-defined Ar/Ar plateau ages of 69.1 ± 1.3, 58 ± 10, and 66.4 ± 1 million years, respectively; these data indicate that a younger multiphase basic magma was injected into a partially crystallized monzonitic magma chamber. The basic intrusions added heat to the system and gave rise to the re-fusion of the already crystallized parts of the monzonitic melt, forming the younger leucogranitic magma. The gradational contacts, cross-cutting relationships, trace element contents, trace element patterns, rare-earth element (REE) patterns, and Ar/Ar geochronological data of the studied igneous suite clearly demonstrate that the acidic and basic rocks of the KMC were contemporaneous and are produced by partial melting of distinct sources rather than by fractional crystallization of a single source.
    International Geology Review - INT GEOL REV. 01/2012;
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    ABSTRACT: The Miocene granitoid plutons exposed in the footwalls of major detachment faults in the Menderes core complex in western Anatolia represent syn-extensional intrusions, providing important geochronological and geochemical constraints on the nature of the late Cenozoic mag-matism associated with crustal extension in the Aegean province. Ranging in composition from granite, granodiorite to monzonite, these plutons crosscut the extensional deformation fabrics in their metamorphic host rocks but are foliated, mylonitized and cataclastically deformed in shear zones along the detachment faults structurally upward near the surface. Crystallization and cooling ages of the granitoid rocks are nearly coeval with the documented ages of metamorphism and deformation dating back to the latest Oligocene– early Miocene that record tectonic extension and exhumation in the Menderes massif. The Menderes granitoids (MEG) are represented by mainly metaluminous-slightly peraluminous, high-K calc-alkaline and partly shoshonitic rocks with their silica contents ranging from 62.5 to 78.2 wt%. They display similar major and trace element characteristics and overlapping inter-element ratios (Zr/Nb, La/Nb, Rb/Nb, Ce/Y) suggesting common melt sources. Their enrichment in LILE, strong negative anomalies in Ba, Ta, Nb, Sr and Ti and high incompatible element abundances are consistent with derivation of their magmas from a subduction-metasomatized, heterogeneous sub-continental lithospheric mantle source. Fractional crystalization processes and lower to middle crustal contamination also affected the evolution of the MEG magmas. These geochemical characteristics of the MEG are similar to those of the granitoids in the Cyclades to the west and the Rhodope massif to the north. Partial melting of the subduction-metasomatized lithospheric mantle and the overlying lower-middle crust produced the MEG magmas starting in the late Oligocene–early Miocene. The heat and the basaltic material to induce this partial melting were provided by asthenospheric upwelling caused by lithospheric delamination. Rapid slab rollback of the post-Eocene Hellenic subduction zone may have peeled off the base of the subcontinental lithosphere, triggering the inferred lithospheric delamination. Both slab retreat-generated upper plate deformation and mag-matically induced crustal weakening led to the onset of the Aegean extension, which has migrated southward through time. The Aegean extensional province is a rapidly deforming and seismically active domain in the Africa–Eurasia convergent zone in the eastern Mediterranean region and is considered to have evolved as a backarc tectonic environment above the north-dipping Hellenic subduction zone (Fig. 1; Le Pichon & Angelier 1979; Jolivet 2001; Faccenna et al. 2003; van Hinsbergen et al. 2005; Jolivet & Brun 2008). Southward retreat of the sub-ducting African lithosphere along the Hellenic trench and the faster SW motion of the southern part of the Anatolian block in the upper plate have resulted in approximately north–south extension in the Aegean region since the Oligo-Miocene (Jolivet et al. 1994; Jolivet & Faccenna 2000; Ring & Layer 2003). The thrust front associated with this subduction zone and its slab retreat has also migrated from the Hellenic trench to the south of the Mediterranean Ridge since then (Le Pichon et al. 2003). These observations suggest that the driving forces for regional extensional tec-tonics in the broader Aegean region reside mainly within the retreating lithospheric slab. Subduction of the Tethyan mantle lithosphere northward beneath Eurasia was nearly continuous since the latest Cretaceous, only temporarily punctuated by the collisional accretion of several ribbon continents (i.e. Pelagonia, Sakarya, Anatolide –Tauride) to the southern margin of Eurasia and related slab breakoff events (Rosenbaum et al. 2002; van Hinsbergen et al. 2005; Dilek & Altunkaynak 2007). Exhuma-tion of middle to lower crustal rocks and the formation of extensional metamorphic domes occurred in the backarc region of this progressively southward migrated trench and the Tethyan slab throughout the Cenozoic.
    Geological Society London Special Publications 01/2009; 321:197-223. · 2.58 Impact Factor