Search

A large drop in seawater [sup.87]Sr/[sup.86]Sr during the Middle Ordovician was among the most rapid in the entire Phanerozoic. New [sup.87]Sr/[sup.86]Sr measurements from Nevada indicate that the rapid shift began in the Pygodus serra conodont zone of the upper Darriwilian Stage. We use a numerical model to explore the hypothesis that volcanic weathering provided the flux of nonradiogenic Sr to the oceans. A close balance between volcanic outgassing and C[O.sub.2] consumption from weathering produced steady pC[O.sub.2] levels and climate through the middle Katian, consistent with recent Ordovician paleotemperature estimates. In the late Katian, outgassing was reduced while volcanic weathering continued, and resulted in a cooling episode leading into the well-known end-Ordovician glaciation.

The crystalline core of the southern Black Hills, South Dakota, exposes an extensive, low-P-high-T aureole of garnet- to second-sillimanite-zone schists centered on the plutonic core of the 1715 Ma Harney Peak Granite (HPG). This paper demonstrates regional patterns of apparent ages observed for 52 [sup.40]Ar/[sup.39] Ar dates of muscovite and biotite in diverse rocks from across the ~1000 [km.sup.2] metamorphic aureole and its plutonic center. About 20 biotite dates, sampled mostly near faults, are influenced by excess [sup.40]Ar and obscure the regional trends. The remaining mica dates reveal radial patterns of apparent younging from outer aureole toward inner granite, with previously unrecognized, elliptical age zones centered on the main HPG pluton and its outliers. The regional pattern of [sup.40]Ar/[sup.39] Ar cooling ages indicates non-uniform slow cooling of the mid-crust between ~1600-1250 Ma. This scenario of delayed slow cooling from aureole to pluton is consistent with published cooling ages for muscovite (Rb/Sr) and apatite (U/Pb), which range from 1690 to 1550 Ma and from 1700 to ~1500 Ma, respectively. To explain these results, it is likely that ambient pre-granite temperatures of the country rocks were >350[degrees]C at the ~10-14 km depth of granite emplacement, as previously proposed, and that the entire complex resided at this depth and cooled slowly from aureole to granite for hundreds of millions of years. Alternatively, or in addition, the HPG and inner aureole were not uplifted until ~1480-1330 Ma, whereupon they finally cooled through ~300-350[degrees]C. Keywords: Geochronology, micas, high-temperature studies, argon retentivity of micas (fieldbased), radiogenic isotopes, argon, strontium, and lead, major and minor elements, pegmatites

Abstract: The Laramide Black Hills uplift of southwest South Dakota exposes a Precambrian crystalline core of ~2560-2600 Ma basement granitoids nonconformably overlain by two Paleoproterozoic intracratonic rift successions. In the [...]

A metapelite from the easternmost Wyoming craton (Black Hills, South Dakota) has been analyzed by microstructural methods to unravel polyphase deformational history associated with 1800-1700 Ma assembly of southern Laurentia. Three deformational fabrics are recognized in oriented thin sections: an ENE-trending [S.sub.1] fabric, preserved as oblique inclusion trails in garnet porphyroblasts; a NNW-trending [S.sub.2] fabric, preserved as microlithons in the rock matrix; and a flattening fabric, [S.sub.3], which transposed [S.sub.1]-[S.sub.2] and dominates the matrix. A complex monazite porphyroblast has been analyzed in situ with the electron microprobe (Ultrachron) to constrain the timing of [S.sub.1]-[S.sub.3] fabric formation associated with monazite growth. The core of this grain uniquely preserves the [S.sub.1]-[S.sub.2] fabrics as sigmoidal inclusion trails. The mean total-Pb age of this domain is 1750 [+ or -] 10 Ma (all dates reported at 95% confidence; n = 39 spots), which is equivalent to the published [sup.207]ph/[sup.206]Pb age for the same domain. These results validate the total-Pb dating method in general and the Ultrachron in particular, for reliable age determination in low-Th monazite, and are interpreted as 1750 Ma minimum ages for the [S.sub.1]-[S.sub.2] fabrics and sequential, [D.sub.1]-[D.sub.2] collisional events that imposed them (~N-directed arc accretion and ~E-W continental collision, respectively). A higher-Th, Y rim of this same 'Rosetta' grain truncates the [S.sub.1]-[S.sub.2] sigmoid, and is associated with resorption textures in garnet porphyroblasts, coupled release of Y, and an [S.sub.3] fabric that pervasively overprinted [S.sub.1]-[S.sub.2] in the rock matrix. The mean Ultrachron date of this domain is 1692 [+ or -] 5 Ma (n = 17 spots), which is slightly younger that the published isotopic age for all monazite rims combined. These results support a ~1715-1690 Ma timeframe for localized doming ([D.sub.3]) related to granite magmatism, the onset of which has been dated independently at 1715 [+ or -] 3 Ma. The timing of post-[D.sub.3] cooling through 350 and 300 [degrees]C is constrained by [sup.40]Ar/[sup.39]Ar dates of ~1610 and ~1480 Ma obtained for separates of [D.sub.3] matrix muscovite and biotite, respectively, which are interpreted as closure ages. This study shows that fabrics in poly-deformed rocks can be dated by linking monazite spot ages to key microtextures. Further, the results of this micrometer-scale study enhance previous knowledge of local thermotectonism (Black Hills) and regional terrane assembly (Laurentia).

The age and mode of formation of the South Caspian Basin are disputed. An ~10-km-thick, predominantly middle Eocene elastic and volcanic succession is exposed in the Talysh mountains of Azerbaijan at its western margin. Here, high-K alkali basalts pass laterally to the east and southeast into volcanogenic sandstone-dominated turbidity current and debris-flow deposits. These southeasterly directed depositional systems accumulated in water depths generally greater than 200 m and fed directly into the western South Caspian Basin. New Ar-Ar ages cluster around 39 Ma, with an upper, 1400-m-thick volcanic interval being deposited in 2.2 [+ or -] 0.2 m.y. We interpret that this rapid deposition and magmatism records a major back-arc extensional/transtensional event in the Talysh, north of the north-dipping Neotethyan subduction zone. This event is recognized across much of southwest Asia and may indicate a period of significant basin formation within the adjacent South Caspian Basin. A transition into Upper Eocene-Lower Oligocene strata, dominated by fine-grained turbidity current and hemipelagic sediments with slope instability features, is interpreted to mark the end of rifting and volcanism in the Talysh and the start of the Arabia-Eurasia collision. Overlying Oligocene coarse clastic rocks are interpreted as the erosional products of localized topography created by the further propagation of compressional deformation into the Talysh region. Keywords: Talysh, South Caspian Basin, rifting, Paleogene, Tethys, Arabia-Eurasia collision.

A question regarding the 1900-1600 Ma assembly of Laurentia is whether the Wyoming province (west-central United States) was part of the Hearne province (west-central Canada) prior to the Early Proterozoic Trans-Hudson orogeny, or a separate entity welded later to a Hearne-Superior continent (central Canada). New 40Ar/39Ar mica dates help to address this question by extending a Middle Proterozoic geochronologic front, long established along the southern Wyoming province, into the Black Hills of South Dakota. This suggests that previously unexplained, north-directed fold nappes ([F.sub.1]) in the Black Hills resulted from island-arc accretion to the south ca. 1780 Ma. North-northwest-trending upright [F.sub.2] folds, which formed during east-west collision of the Wyoming and Superior provinces, must therefore be younger. New 40Ar/39Ar horn-blende dates, recently published age data, and crustal heat-flow considerations further suggest that this collision began at or before ca. 1770 Ma and culminated with posttectonic magmatism beginning ca. 1715 Ma (the Harney Peak granite). This tectonic-magmatic interval is [approximately]50-60 m.y. younger than that reported for the Hearne-Superior collision (Trans-Hudson orogeny in Canada). Comparably young metamorphic dates (1810-1710 Ma) also typify the eastern and northern Wyoming province periphery (western Dakotas and southwestern Montana). Collectively, these data suggest that the Hearne and Wyoming provinces were once separate continents that were ultimately welded to the Superior province (and to each other) during distinct Early Proterozoic orogenies. Regional relationships further suggest that final docking of the eastern Wyoming province with Laurentia began during the ca. 1780-1740 Ma interval of island-arc accretion along the southern margin of the growing craton.

Presents a direct dating of mid-Cretaceous alteration of the Kirkpatrick Basalt in northern Victoria Land, Antarctica. Study of the samples from the uppermost flow unit of the Kirkpatrick Basalt; Paleomagnetic pole positions for the basalts and dolerites; Alteration in northern Victoria Land.

A large drop in seawater 87Sr/86Sr during the Middle Ordovician was among the most rapid in the entire Phanerozoic. New 87Sr/86Sr measurements from Nevada indicate that the rapid shift began in the Pygodus serra conodont zone of the upper Darriwilian Stage. We use a numerical model to explore the hypothesis that volcanic weathering provided the flux of nonradiogenic Sr to the oceans. A close balance between volcanic outgassing and CO2 consumption from weathering produced steady pCO2 levels and climate through the middle Katian, consistent with recent Ordovician paleotemperature estimates. In the late Katian, outgassing was reduced while volcanic weathering continued, and resulted in a cooling episode leading into the well-known end-Ordovician glaciation. [ABSTRACT FROM AUTHOR]

Within extended orogens, records that reflect the driving processes and dynamics of early extension are often overprinted by subsequent orogenic collapse. The Copper Mountains of northeastern Nevada preserve an exceptional record of hinterland extensional deformation and high-elevation basin formation, but current geochronology and thermochronology are insufficient to relate this to broader structural trends in the region. This extension occurred concurrent with volcanism commonly attributed to Farallon slab removal. We combine thermochronology of both synextensional hanging-wall strata and footwall rocks to comprehensively evaluate the precise timing and style of this deformation. Specifically, we apply (U-Th)/(He-Pb) double dating of minerals extracted from Eocene-Oligocene Copper Basin strata with multi-mineral (U-Th)/He and 40Ar/39Ar thermochronology of rocks sampled across an ~20 km transect of the Copper Mountains. We integrate basement and detrital thermochronology records to comprehensively evaluate the timing and rates of hinterland extension and basin sedimentation. Cooling and U-Pb crystallization ages show the Coffeepot Stock, which spans the width of the Copper Mountains, was emplaced at ca. 109-108 Ma, and then cooled through the 40Ar/39Ar muscovite and biotite closure temperatures by ca. 90 Ma, the zircon (U-Th)/He closure temperature between ca. 90 and 70 Ma, and the apatite (U-Th)/He closure temperature between 43 and 40 Ma. Detrital apatite and zircon (U-Th)/(He-Pb) double dating of late Eocene fluvial and lacustrine strata of the Dead Horse Formation and early Oligocene fluvial strata of the Meadow Fork Formation, both deposited in Copper Basin, shows that Early Cretaceous age detrital grains have a cooling history that is analogous to proximal intrusive rocks of the Coffeepot Stock. At ca. 38 Ma, cooling and depositional ages for Copper Basin strata reveal rapid exhumation of proximal source terranes (cooling rate of ~37 °C/m.y.); in these terranes, 8-12 km of slip along the low-angle Copper Creek normal fault exhumed the Coffeepot Stock in the footwall. Late Eocene-early Oligocene slip along this fault and an upper fault splay, the Meadow Fork fault, created a half graben that accommodated ~1.4 km of volcaniclastic strata, including ~20 m of lacustrine strata that preserve the renowned Copper Basin flora. Single-crystal sanidine 40Ar/39Ar geochronology of interbedded tuffs in Copper Basin constrains the onset of rapid exhumation to 38.0 ± 0.9 Ma, indicating that surface-breaching extensional deformation was coincident with intense proximal volcanism. Coarse-grained syndeformational sediments of the Oligocene Meadow Fork Formation were deposited just prior to formation of an extensive regional Oligocene-Miocene unconformity and represent one of the most complete hinterland stratigraphic records of this time. We interpret this history of rapid late Eocene exhumation across the Copper Mountains, coeval volcanism, and subsequent unconformity formation to reflect dynamic and thermal effects associated with Farallon slab removal. The final phase of extension is recorded by late, high-angle normal faults that cut and rotate the early middle Miocene Jarbidge Rhyolite sequence, deposited unconformably in the hanging wall. These results provide an independent record of episodic Paleogene to Miocene exhumation documented in Cordilleran metamorphic core complexes and establish that substantial extension occurred locally in the hinterland prior to province-wide Miocene extensional break-up. [ABSTRACT FROM AUTHOR]

Recovery from the Late Permian mass extinction was slowed by continued environmental perturbations during the Early Triassic. Rapid fluctuations of the Early Triassic marine carbonate carbon isotope record indicate instability in the global carbon cycle, and recent δ18Oapatite studies link elevated temperatures to the prolonged biotic recovery. High temperatures potentially caused enhanced continental weathering that was detrimental to marine ecosystems, but linking weathering rates to temperature has proven difficult. One proxy for weathering is the 87Sr/86Sr of marine carbonate; we present here an 87Sr/86Sr record from an upper Permian-lower Triassic succession near Zal, Iran, that is coupled to a δ13Ccarbonate record. An increase in the rate of 87Sr/86Sr rise from the Dienerian to the Smithian may be linked to elevated continental weathering rates caused by warming during the Smithian. [ABSTRACT FROM AUTHOR]

Abstract: During the Eocene, the northern Neo-Tethys suture zone and immediately adjacent areas in NW Turkey were loci of emplacement of a number of granitoid plutons with age relations and geochemical signatures indicative of magma generation in association with plate convergence. Discrete granitoid plutons of Eocene age in this region are exposed mainly in two distinct belts within and north of the northern Neo-Tethys suture zone. We report new SHRIMP U–Pb zircon and 40Ar/39Ar geochronology along with Sr–Nd isotope data from the granitoids in order to constrain their melt source(s), cooling history, and tectono-magmatic evolution with respect to the Early Cenozoic collisional tectonics of the region. Ranging in composition from diorite to granite, the plutons are largely represented by medium- to high-K calc-alkaline, I-type granites accompanied by minor amounts of shoshonitic rocks with syenite compositions and mafic microgranular enclaves in some of the plutons. In terms of trace element systematics the granitoids and the associated enclaves from the entire Eocene suite display close similarities to magmas from subduction-related or active continental margin settings, characterized by significant enrichment in LILE/HFSE relative to MORB. The rocks display significantly heterogeneous distributions of radiogenic isotopes with 87Sr/86Sr and 143Nd/144Nd ranging from 0.705824 to 0.708363 and from 0.512384 to 0.512718 respectively, suggesting multi-component melt interaction in their genesis. The granitoids from the southern and northern belts yielded zircon SHRIMP U–Pb ages of 52.8±1.8 to 45.41±0.34Ma and 47.02±0.82 to 36.79±0.67Ma, respectively, indicating overlapping emplacement ages, with some relatively younger ages from the northern belt. 40Ar/39Ar dating of biotite and hornblende also yielded similarly overlapping cooling ages (51.8±0.1 to 44.9±0.2Ma and 45.3±0.1 to 36.0±0.1Ma, respectively). Our combined geochronological data from the co-genetic zircon, and hornblende and biotite separates reveals rapid cooling of the plutons immediately after emplacement, which can readily be attributed to rapid uplift starting at∼53Ma. Unlike the intrusions in the southern belt, some of the granitoids in the northern belt contain inherited zircons of older ages, possibly reflecting more pronounced effects of crustal input in their genesis. On the basis of new geochemical and geochronological data, we infer that melt generation to produce the granitic plutons shortly after subduction waned in the region is most probably related to abrupt thermal weakening of the orogenic lithosphere due either to slab break-off or partial removal of a thermal boundary layer between lithosphere and asthenosphere that led to syn-convergent extension and collapse in the overriding plate, resulting in the onset of the development of the Aegean extensional province. [Copyright &y& Elsevier]