Detrital zircon and apatite (U‐Th)/He geochronology of intercalated baked sediments: a new approach to dating young basalt flows

Simple numerical models suggest that many basaltic lava flows should sufficiently heat the sediments beneath them to reset (U‐Th)/He systematics in detrital zircon and apatite. This result suggests a useful way to date such flows when more conventional geochronological approaches are either impractical or yield specious results. We present here a test of this method on sediments interstratified with basalt flows of the Taos Plateau Volcanic Field of New Mexico. Nineteen zircons and apatites from two samples of baked sand collected from the uppermost 2 cm of a fluvial channel beneath a flow of the Upper Member of the Servilleta Basalt yielded an apparent age of 3.487 ± 0.047 Ma (2 SE confidence level), within the range of all published 40Ar/39Ar dates for other flows in the Upper Member (2.81–3.72 Ma) and statistically indistinguishable from the 40Ar/39Ar dates for basal flows of the Upper Member with which the studied flow is broadly correlative (3.61 ± 0.13 Ma). Given the high yield of 4He from U and Th decay, this technique may be especially useful for dating Pleistocene basalt flows. Detailed studies of the variation of (U‐Th)/He detrital mineral dates in sedimentary substrates, combined with thermal modeling, may be a valuable tool for physical volcanologists who wish to explore the temporal and spatial evolution of individual flows and lava fields.</p

Amino acid racemization dating of marine shells: a mound of possibilities

Shell middens are one of the most important and widespread indicators for human exploitation of marine resources and occupation of coastal environments. Establishing an accurate and reliable chronology for these deposits has fundamental implications for understanding the patterns of human evolution and dispersal. This paper explores the potential application of a new methodology of amino acid racemization(AAR) dating of shell middens and describes a simple protocol to test the suitability of different molluscanspecies. This protocol provides a preliminary test for the presence of an intracrystalline fraction of proteins(by bleaching experiments and subsequent heating at high temperature), checking the closed system behaviour of this fraction during diagenesis. Only species which pass both tests can be considered suitable for further studies to obtain reliable age information. This amino acid geochronological technique is also applied to midden deposits at two latitudinal extremes: Northern Scotland and the Southern Red Sea.Results obtained in this study indicate that the application of this new method of AAR dating of shells has the potential to aid the geochronological investigation of shell mounds in different areas of the world

SHRIMP ion probe zircon geochronology and Sr and Nd isotope geochemistry for southern Longwood Range and Bluff Peninsula intrusive rocks of Southland, New Zealand

Permian–Jurassic ultramafic to felsic intrusive complexes at Bluff Peninsula and in the southern Longwood Range along the Southland coast represent a series of intraoceanic magmatic arcs with ages spanning a time interval of 110 m.y. New SHRIMP U-Pb zircon data for a quartz diorite from the Flat Hill complex, Bluff Peninsula, yield an age of 259 ± 4 Ma, consistent with other geochronological and paleontological evidence confirming a Late Permian age. The new data are consistent with an age of c. 260 Ma for the intrusive rocks of the Brook Street Terrane. SHRIMP U-Pb zircon ages for the southern Longwood Range confirm that intrusions become progressively younger from east to west across the complex. A gabbro at Oraka Point (eastern end of coastal section) has an age of 245 ± 4 Ma and shows virtually no evidence of zircon inheritance. The age is significantly different from that of the Brook Street Terrane intrusives. Zircon ages from the western parts of the section are younger and more varied (203–227 Ma), indicating more complex magmatic histories. A leucogabbro dike from Pahia Point gives the youngest emplacement age of 142 Ma, which is similar to published U-Pb zircon ages for the Anglem Complex and Paterson Group on Stewart Island

The geology and geochronology of Al Wahbah maar crater, Harrat Kishb, Saudi Arabia

Al Wahbah is a large (∼2.2 km diameter, ∼250 m deep) maar crater in the Harrat Kishb volcanic field in western Saudi Arabia. It cuts Proterozoic basement rocks and two Quaternary basanite lava flows, and is rimmed with an eroded tuff ring of debris from the phreatomagmatic explosion that generated the crater. A scoria cone on the northern wall of the crater was dissected by the explosion and exposes a dolerite plug that was intruded immediately prior to crater formation. The dolerite plug yields a &lt;sup&gt;40&lt;/sup&gt;Ar/&lt;sup&gt;39&lt;/sup&gt;Ar age of 1.147 ± 0.004 Ma. This is the best possible estimate of the time Al Wahbah crater formed. It is a few tens of thousand years younger than the age of the lower and upper basalt flows, 1.261 ± 0.021 Ma and 1.178 ± 0.007 Ma respectively. A dolerite dyke exposed within the basement in the wall of the crater is dated at 1.886 ± 0.008 Ma. This is the most precise age so far determined for the initiation of basaltic volcanism of Harrat Kishb, and confirms that it is significantly younger than the other post-rift volcanic provinces in the region. This study provides constrains the timing of humid climatic conditions in the region and suggests that the Quaternary basaltic volcanism that stretches the length of the western side of the Arabian peninsula may prove to be useful for establishing palaeoclimatic conditions

Tectonic history of the South Tannuol Fault Zone (Tuva region of the northern Central Asian Orogenic Belt, Russia) : constraints from multi-method geochronology

In this study, we present zircon U/Pb, plagioclase and K-feldspar Ar-40/Ar-39 and apatite fission track (AFT) data along the South Tannuol Fault Zone (STFZ). Integrating geochronology and multi-method thermochronology places constraints on the formation and subsequent reactivation of the STFZ. Cambrian (similar to 510 Ma) zircon U/Pb ages obtained for felsic volcanic rocks date the final stage of STFZ basement formation. Ordovician (similar to 460-450 Ma) zircon U/Pb ages were obtained for felsic rocks along the structure, dating their emplacement and marking post-formational local magmatic activity along the STFZ. Ar-40/Ar-39 stepwise heating plateau-ages (similar to 410-400 Ma, similar to 365 and similar to 340 Ma) reveal Early Devonian and Late Devonian-Mississippian intrusion and/or post-magmatic cooling episodes of mafic rocks in the basement. Permian (similar to 290 Ma) zircon U/Pb age of mafic rocks documents for the first time Permian magmatism in the study area creating prerequisites for revising the spread of Permian large igneous provinces of Central Asia. The AFT dating and Thermal history modeling based on the AFT data reveals two intracontinental tectonic reactivation episodes of the STFZ: (1) a period of Cretaceous-Eocene (similar to 100-40 Ma) reactivation and (2) the late Neogene (from similar to 10 Ma onwards) impulse after a period of tectonic stability during the Eocene-Miocene (similar to 40-10 Ma)

Evidence from Rb–Sr mineral ages for multiple orogenic events in the Caledonides of Shetland, Scotland

Shetland occupies a unique central location within the North Atlantic Caledonides. Thirty-three new high-precision Rb–Sr mineral ages indicate a polyorogenic history. Ages of 723–702 Ma obtained from the vicinity of the Wester Keolka Shear Zone indicate a Neoproterozoic (Knoydartian) age and preclude its correlation with the Silurian Moine Thrust. Ordovician ages of c. 480–443 Ma obtained from the Yell Sound Group and the East Mainland Succession constrain deformation fabrics and metamorphic assemblages to have formed during Grampian accretionary orogenic events, broadly contemporaneously with orogenesis of the Dalradian Supergroup in Ireland and mainland Scotland. The relative paucity of Silurian ages is attributed to a likely location at a high structural level in the Scandian nappe pile relative to mainland Scotland. Ages of c. 416 and c. 411 Ma for the Uyea Shear Zone suggest a late orogenic evolution that has more in common with East Greenland and Norway than with northern mainland Scotland

Topographic Development History of the Alaska Range

he overall goal of this project is to use variations in sediment source through time as a proxy for deciphering the uplift history of the Alaska Range (Fig. 1). In particular, we plan to track variations in sediment provenance through time for the Oligocene to present Tanana Basin. The main possible sediment source regions are north of the Alaska Range, south of the Alaska Range, and from the Alaska Range itself (Fig. 2). Furthermore, we will use the sediment source interpretation to test the hypothesis that the Nenana River changed direction during the Miocene (23 Ma to 5.3 Ma)

Topographic Development History of the Alaska Range

The overall goal of this project is to use variations in sediment source through time as a proxy for deciphering the uplift history of the Alaska Range (Fig. 1). In particular, we tracked variations in sediment provenance through time for the Oligocene to present Tanana Basin. The three main sediment source regions are north of the Alaska Range, south of the Alaska Range, and from the Alaska Range itself (Fig. 2). Furthermore, we will use the sediment source interpretation to test the hypothesis that the Nenana River changed direction during the Miocene (23 Ma to 5.3 Ma) (e.g. Brennan, 2012