Geochemistry Of Plutonic Rocks Research Articles

Petrology and geochemistry of plutonic rocks in the Northwest Pacific Ocean and their geodynamic interpretation

The paper presents data on the petrology and geochemistry of plutonic rocks dredged from the Stalemate Fracture Zone, Northwest Pacific Ocean, during Cruise SO201-1 of the R/V “Sonne”. We proposed also the reconstruction of their formation conditions and interpretation of their tectonic evolution. The genesis of gabbroids found among plutonic rocks composing the Cretaceous–Paleogene basement of the northwestern part of the Pacific Ocean was related to magmatism at the ancient spreading center and provides record of the evolution of the parental magmatic melts of N-MORB. Along with related peridotites, basalts, and dolerites, these rocks can be attributed to the disintegrated the Cretaceous–Paleogene oceanic lithosphere of the Pacific Ocean. The shallow mantle beneath the ancient oceanic crust of this area is made up of depleted magmatic spinel lherzolite, harzburgite, and dunite. The fact that gabbro-diorite and diorite that are not genetically related to the rocks of the Cretaceous–Paleogene basement of the Northwest Pacific occur at the eastern termination of the Stalemate Fracture Zone possibly reflects the complicated structure of the tectonic collage of rocks of different age that were produced in different geodynamic environments and were later tectonically brought together near the frontal portion of the Aleutian island arc. Judging by the isotopic–geochemical characteristics of these rocks, they cannot be classed with the family of oceanic plagiogranites. Deformations of the oceanic basement can be discerned throughout the whole Stalemate Fracture Zone as brecciation and large-amplitude vertical displacements within the oceanic lithosphere.

RbSr geochronology and geochemistry of plutonic rocks from the Wadi Shuqub quadrangle, west-central Arabian Shield

New geochemical and RbSr isotopic age data are presented for three suites of plutonic rocks from the central and western parts of the Wadi Shuqub quadrangle. The syn-tectonic diorite-tonalite are the products of early arc magmatism and the post-tectonic biotite-granite is the result of intra-arc plutonism. These plutonic suites have an intrunsive relationship with the layered metavolcanic and metasedimentary rocks of the Baish and Bahah groups, which have contributed to the development of an oceanic crust. The syn-tectonic tonalite, which forms a large batholith of tonalite-quartz diorite in the western part of the Wadi Shuqub quadrangle, yields an age of 854 ± 10 Ma (2 σ) with an initial 87Sr/ 86Sr ratio of 0.70159 ± 0.00004 and an MSWD value of 1.97. The late-tectonic diorite-tonalite stock yields an age of 815 ± 13 Ma (2 σ) with an initial 87Sr/ 86Sr ratio of 0.7029 ± 0.00005 (2 σ) and an MSWD value of 1.81. The post-tectonic suite of biotite-granite from the Gabalat area defines a poorly fitted Rb-Sr isochron age of 552 ± 20 Ma with an initial 87Sr/ 86Sr ratio of 0.71327 ± 0.003 (2 σ) and an MSWD value of 4.47. The syn- to late-tectonic tonalite and diorite-tonalite are of low-K calc-alkaline affinity and are characterized by low abundances of K, Rb/Sr, and high Sr, CaO, MgO, TiO 2, and FeO t (total Fe as FeO is denoted as FeO t. These two suites with low initial 87Sr/ 86Sr ratios are likely to be derived from a mantle differentiation or partial melting of the basic rocks above the subduction zone. The post-tectonic biotite-granite is enriched in SiO 2, K, Rb, Rb/Sr and depleted in Sr, CaO, MgO and FeO t and is of dominantly high-K calc-alkaline affinity. This suite of biotite-granite with high initial 87Sr/ 86Sr ratio appears to have been derived from lower crustal material. Limited trace element data on diorite-tonalite and biotite-granite show that the biotite-granite is enriched in Rb, Pb, Zn, Sn, U, Th, Zr, and Y and depleted in Sr, Ba, Cu and Ni compared with diorite-tonalite.