Melting Of Oceanic Lithosphere Research Articles

Adakite genesis and plate convergent process: Constraints from whole rock and mineral chemistry, Sr, Nd, Pb isotopic compositions and U-Pb ages of the Lakhshak magmatic suite, East Iran

The genesis of Eocene to Early Oligocene plutons and dykes emplaced in the suture zone between two colliding continental blocks in southeastern Iran has been the subject of debate concerning the processes that formed magmas and influenced their intrusive history. We report geological, geochemical, mineralogical and isotope evidence that constrains the origin of the Lakhshak plutonic complex. SiO 2 contents in the granodioritic pluton and dacite dykes average 67 wt%, and in the monzodiorite dykes 53 wt%. Based on (La/Yb) N vs. Yb N the rocks correspond to adakites, originating from 10 to 25% melting of garnet amphibolite. The high A1 2 O 3 (14.95–17.15 wt%), Sr (442–1385 ppm) and Ba (601–1935 ppm) contents and relatively high La/Yb (25–45) and Sr/Y (35–57), but low Yb (mostly <1.6 ppm) and Y (mostly <18.2 ppm) are also features typical of adakitic rocks. Zircon and titanite from two samples of the pluton yield U-Pb ages of 29.98 ± 0.08 and 29.90 ± 0.05 Ma, a monzodiorite dyke yields 29.02 ± 0.10 Ma, and a dacite dyke 29.39 to 27.96 Ma. These ages confirm that the magmatism was post-collisional. The monzodiorite and dacite dykes have ( 87 Sr/ 86 Sr) i = 0.7053–0.7056 and εNd i = −1.1 to 2.2 (except one sample with εNd i = −8.6), which limit the extent of crustal assimilation and plot along a mixing array between Sistan ophiolites and the country rocks of the pluton. This is consistent with an origin from oceanic slab melts variously mixed with melts from flysch sediments. Isotope compositions of Sr, Nd and Pb, xenocrystic zircons, and magma mixing and mingling textures support such a process. Thermobarometry on various minerals indicates a continuum of crystallization stages during magma ascent, with pyroxene formed at 8 kbar, followed by amphiboles, which record conditions decreasing from 1166 °C / 7.8 kbar to 933 °C / 4.0 kbar. Tectonically, the Lakhshak adakites were most likely the consequence of post-collisional delamination that led to melting of Sistan oceanic lithosphere and its interaction with melts from pelagic sediments. • Lakhshak suite includes a granodiorite pluton and dacitic and monzodioritic dykes. • Intruded at 30 to 28 Ma after closing of Sistan Ocean and continental collision • It corresponds to adakites, originating from 10 to 25% melting of garnet amphibolite. • Caused by delamination, melting of oceanic lithosphere and some melted sediments

Craton formation in Late Archean subduction zones revealed by first Greenland eclogites

It is now well established that the early continental crust was formed by melting of basaltic lithologies such as amphibolite and eclogite. However, considerable uncertainty surrounds the geologic environment in which melting took place. Commonly invoked options range between melting at the underside of oceanic plateaus above mantle plumes or melting of oceanic lithosphere during shallow subduction. Distinguishing between these scenarios has important implications for the early evolution of continents. We use the first eclogites discovered from the North Atlantic craton (NAC) to constrain the formation of the deep root to this continent. Late Archean eclogite xenoliths (2.7 ± 0.3 Ga) from a kimberlite in West Greenland are broadly coeval with a major regional episode of tonalite-trondhjemite-granodiorite (TTG) magmatism. Major and trace element systematics of the eclogites reveal a highly refractory character that is mirrored by NAC peridotites. Moreover, the refractory eclogites define a complementary relationship to the Late Archean TTG granitoids from the NAC, and their elevated garnet δ 18 O values along with negative Eu anomalies suggest seafloor-altered oceanic crust as the most viable eclogite protolith. These results from Greenland provide strong support for a model in which early continental crust grew by melting of basaltic slabs in subduction zones, where tectonic stacking of down-going oceanic lithosphere provided the mechanism that coupled formation of cratonic crust and mantle.

Geology of the Moonmera porphyry deposit, Queensland, Australia

The Moonmera deposit in central coastal Queensland is thus far the oldest documented porphyry deposit on the margin of the Pacific Ocean. Age of mineralization based on this investigation is 245 m.y. The deposit is associated with late-stage porphyritic differentiates of the Bouldercombe Complex, a late Permian granodioritic body of batholithic dimensions. This complex was emplaced into an east-west dilatant zone developed across the Yarrol Basin at the close of the Permo-Triassic Hunter-Bowen orogeny. The members of the intrusive complex comprise an early granodiorite porphyry, followed by an andesitic tuffisite pipe, and finally the youngest granodiorite porphyry, with associated radial and ring-dike sets. Equigranular quartz diorite of the Boulder-combe Complex forms about 95 percent of the deposit9s wall rocks.Sulfide mineralization took place during three principal episodes that are time equivalents of the three members of the Moonmera intrusive complex. Stage 1 mineralization is pyrite-chalcopyrite-molybdenite that preceded the oldest granodiorite porphyry and is volumetrically the most important mineralization at Moonmera. This mineralization which is principally in steeply dipping fractures and veins in quartz diorite has a well-developed zonation surrounding the intrusive complex. The zones are a low-grade core, a chalcopyrite-molybdenite inner halo, a chalcopyrite-pyrite intermediate halo, and a pyrite outer halo. Stage 2 mineralization occurs as very finely disseminated chalcopyrite in the matrix of the tuffisite pipe. Stage 3 mineralization is disseminated chalcopyrite either in the core of a young granodiorite porphyry pluton or in its uppermost roof zone. It also includes massive chalcopyrite and pyrite fillings of voids in breccias that mantle the roof zones of individual young intrusions.Alteration has taken place during three equivalent episodes. Alteration accompanying stage 1 mineralization is composed of a weakly developed zone of pervasive biotite which overlies a deeper zone of moderate biotite and anhydrite. Secondary orthoclase in veins, and selvages and envelopes to veins, occurs in both the shallow and deep biotite zones. Strong biotitization of the older granodiorite porphyry groundmass also occurred during this episode. Alteration accompanying stage 2 mineralization was predominantly strong biotitization of the tuffisite matrix. The episode accompanying stage 3 mineralization includes moderate to intense sericitization of the cores of the younger granodiorite porphyry intrusions and their capping breccias and propylitization of the outer margins of these intrusions.The parental magma of the Bouldercombe Complex and the Moonmera intrusive complex is believed to be derived by partial melting of oceanic lithosphere that was subducted beneath the eastern margin of Queensland during the late Permian.