500 m.y. of thermal history elucidated by multi-method detrital thermochronology of North Gondwana Cambrian sandstone (Eilat area, Israel)

Following the Neoproterozoic Pan-African orogeny, the Arabian-Nubian Shield of North Africa and Arabia was eroded and then covered by Cambrian sandstones that record the onset of platform sedimentation. We applied K-feldspar 40Ar/39Ar, zircon and apatite fission-track, and apatite (U-Th)/He thermochronology to detritus from Cambrian sandstones of southern Israel deposited at ca. 500 Ma. U-Pb detrital zircon ages from these sandstones predate deposition and record the earlier Neoproterozoic crustal evolution of the Pan-African orogens. 40Ar/39Ar ages from 50 single grains of K-feldspar yield a Cambrian mean of ca. 535 Ma. The 40Ar/39Ar age spectrum of a multi-grain K-feldspar aliquot displays diffusion behavior compatible with >560 Ma cooling later affected by a heating event. Assuming that the high-temperature domains of the K-feldspars have not been affected by subsequent (hydro)thermal events, and taking previously published K-Ar and Rb-Sr ages from other parts of the East African Orogen at face value, these ages apparently record Pan-African thermal resetting below a thick volcano-sedimentary pile similar to the Saramuj conglomerate in Jordan and/or the Hammamat in Egypt. Detrital zircon fission-track (ZFT) ages cluster around 380 Ma, consistent with previous ZFT results from Neoproterozoic basement and sediments of the region, revealing that the Cambrian platform sequence experienced a middle Devonian thermal event and low-grade metamorphism. Regional correlation indicates that during Devonian time the sedimentary cover atop the Cambrian in Israel was never in excess of 2.5 km, requiring an abnormally steep geothermal gradient to explain the complete ZFT annealing. A basal Carboniferous unconformity can be traced from Syria to southern Saudi Arabia, suggesting that the observed Devonian ZFT ages represent a regional tectonothermal event. Similar Devonian ZFT ages were reported from Arabian-Nubian Shield basement outcrops in the Eastern Desert, 500 km south of Eilat. The detrital apatites we studied all have extremely rounded cores suggestive of a distant provenance, but some grains also feature distinct euhedral, U-rich apatite overgrowth rims. Authigenic apatite may have grown during the late Devonian thermal event we dated by ZFT, coinciding with existing Rb-Sr ages from authigenic clays in the same deposits and leading to the conclusion that the Devonian event was probably hydrothermal. Like the ZFT ages, the detrital apatite fission-track (AFT) ages were also completely reset after deposition. Sixty single-grain detrital AFT ages group at ca. 270 Ma with significant dispersion. Inverse modeling of the AFT data indicate extended and/or repeated residence in the AFT partial annealing zone, in turn suggesting an episodic burial-erosion history during the Mesozoic caused by low-amplitude vertical motions. Seven detrital apatite (U-Th)/He ages scatter between 33 and 77 Ma, possibly resulting from extreme compositional zonation associated with the authigenic U-rich overgrowths. The ca. 70 Ma (U-Th)/He ages are more likely to be accurate, setting 1–2 km as an upper limit (depending on the geothermal gradient) on the post-Cretaceous exhumation of the Cambrian sandstone and showing no evidence for substantial denudation related to Tertiary rifting of the Red Sea

40Ar/39 Dates from the Harquahala and Little Harquahala Mountains, west-central Arizona. Part I

This report includes the analytical data for four 40Ar/39Ar cooling dates from samples collected in the Harquahala and Little Harquahala Mountains of west-central Arizona. The samples were collected to constrain the cooling history and timing of mineralization in this area [Richard et aI., 1990; Richard, 1988]. Analyses were done at Stanford University. For each of the 4 samples described here, an incremental thermal release age spectra is included, along with a table summarizing the data used to calculate ages, and a table of raw data from each temperature increment. Argon correlation diagrams are included for each sample, on which the ratio of 4°Ar (radiogenic and atmospheric) to 36Ar is plotted against the ratio 39 ArP6Ar. The regression lines used to calculate isochron ages are indicated on the plots. Regressions were calculated using a simple least squares method. For the hornblende samples, a K/Ca ratio plot is included above the age spectra to provide an indication of the compositional consistency of the degassed material in each temperature increment.Documents in the AZGS Document Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact [email protected]

Neoproterozoic crustal growth at the margin of the East Gondwana continent - age and isotopic constraints from the easternmost inliers of Oman

The Jebel Ja’alan and Qalhat inliers of Oman represent the easternmost exposures in theArabian peninsula of Neoproterozoic basement associated with the East African orogen and the assembly of East and West Gondwana. These inliers expose tonalitic gneisses and metasediments intruded by granodiorites and granites of the Ja’alan batholith. Zircon from the gneisses yield U-Pb SIMS ages of ca. 900-880 Ma, which are interpreted as crystallisation ages. These represent the oldest magmatic events associated with the closure of theMozambique Ocean reported to date. Zircon of this age is also the dominant component in the metasediments. The Ja’alan batholith yields ages of ca. 840-825 Ma. Nd isotopes indicate that both the gneisses and the batholith range from juvenile to slightly more evolved, with εNd(t) of +6 to +1.5 interpreted to reflect variable contamination by older, evolved continental material;this is also indicated by >900 Ma detrital zircon from the metasediments. The Nd data also contrast with the uniformly juvenile signature of younger, ca. 840 Ma, rocks of the Marbat region of southern Oman that lie structurally to the west. The Ja’alan and Qalhat inliers thus document eastward increasing age and continental influence, consistent with progressive development of arc rocks onto the western margin of East Gondwana, although the location and nature of the eastern continental block remains elusive