. (1999). Petrology and geochemistry of late-stage intrusions of the a-type, mid-Proterozoic pikes peak batholith (central Colorado, USA): Implications for petrogenetic models. Precambrian Research,, 271-305. doi:10.1016/ S0301-9268(99) Gabbros and mafic dikes associated with the sodic granitoids have ߳ Nd (1.08 Ga) of −3.0 to +3.5, which are lower than depleted mantle at 1.08 Ga, and their trace element characteristics suggest derivation from mantle sources that were previously affected by subduction-related processes. However, it is difficult to characterize the mantle component in these magmas, because assimilation of crust during magma ascent could also result in their observed geochemical features.The Pikes Peak batholith is composed of at least two petrogenetically different granite types, both of which exhibit geochemical characteristics typical of A-type granites. Models proposed for the petrogenesis of the granitoids imply the existence of mafic rocks at depth and addition of juvenile material to the crust in central Colorado at ~1.1 Ga.3

No abstract

No abstract

The potential petrogenetic link between a crystal-poor rhyolite (the Rhyolite Canyon Tuff) and its associated subvolcanic intrusion and crystal-rich post-caldera lavas from Turkey Creek, Arizona (USA), is examined using zircon chemical abrasion-thermal ionization mass spectrometry U-Pb geochronology and inductively coupled plasma mass spectrometry trace element analyses. U-Pb ages indicate that zircon growth within the rhyolite and the dacite-monzonite porphyry magmas was coeval over ~300 k.y. prior to the large eruptive event. Trends in zircon trace elements (Hf, Y/Dy, Sm/Yb, Eu/Eu*) through time in the dacitic-monzonitic units and rhyolite reflect melt evolution dominated by crystal fractionation. Importantly, the Y/Dy ratio in zircons in both units remains mostly similar for the first ~150 k.y. of the system's evolution, but the dominant population in the rhyolitic unit diverges from that of the dacite-monzonite porphyry ~150 k.y. before eruption. We interpret this divergence in trace element composition to record the assembly time of the melt-rich cap within its intermediate mush zone in the upper crustal reservoir. These results are consistent with (1) a connection between plutonic and volcanic realms in the upper crust, (2) a protracted time scale for constructing an intermediate mush large enough to hold 500 km 3 of rhyolite, and (3) the prolonged extraction of that melt prior to eruption.

New 40 Ar/ 39 Ar, U–Pb, and Sm–Nd data from 13 metamorphic samples constrain the formation and exhumation of the high-pressure Maksyutov Complex in the southern Uralide orogen. The Maksyutov Complex records the highest-pressure metamorphism in the southern Uralides, and is part of the thrust stack that composes the footwall beneath the Main Ural Fault. The new isotopic data from eclogite in the Lower Unit of the Maksyutov Complex include 40 Ar/ 39 Ar data from phengite, U–Pb data from rutile and apatite and Sm–Nd data from garnet, clinopyroxene, rutile and apatite. These data indicate that eclogite–facies metamorphism occurred in the Mid-Devonian Epoch ( c . 380 Ma), corresponding with eastward subduction of the East European craton beneath the Magnitogorsk island arc. After partial exhumation, the Lower unit was juxtaposed c . 360 Ma (based on 40 Ar/ 39 Ar white mica data) with the Middle and Upper units at mid-crustal levels during the development of a D 3 shear zone within the subduction zone. By the Late Carboniferous Epoch, the Maksyutov Complex was exhumed to upper-crustal levels, compatible with sedimentologic evidence that at this time the foreland thrust and fold belt began to form, and material was shed off it into the foreland basin.

No abstract

No abstract

Metasomatized basalts are enclosed in antigorite serpentine mélange of the Upper Unit of the Maksyutov Complex, southern Ural Mountains, Russia. The structurally Lower Unit of this complex consists predominantly of mafic eclogite and sedimentary gneiss metamorphosed at ultrahigh pressures; rocks of the Upper Unit underwent blueschist-facies conditions. The metasomatized rocks are fine-grained blocks, variably composed of chlorite, garnet, lawsonite, white-mica, epidote, and titanite. Partial rodingitization accompanied serpentinization of the surrounding peridotite, and predated the high-pressure metamorphism that led to the formation of lawsonite. Extensive Mg metasomatism followed rodingitization, leading to prevalent Mg-rich rims around the mafic blocks. Potassium metasomatism, although minor compared to the rodingitization and Mg metasomatism, contributed to the formation of muscovite pseudomorphs after lawsonite during exhumation.