Recent work in Barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times. This result is inconsistent with most geodynamic models of crustal over-thickening and conductive heating, wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times. Instead, the presence of addition...

The Omai intrusion-centred Au-quartz vein system, the largest Au producer presently operating in the Guyana Shield, was sampled for detailed U-Pb and Pb-Pb geochronology and petrological investigation. The age of a metavolcanic/sub-volcanic unit in the host rock sequence is 212092 Ma. Zircon analyses from the main body dioritic rocks give U-Pb ages of 209496, 2092911 and 2096+11/−10 Ma. Magmati...

The rise of the Isthmus of Panama, linked to a number of climatic, paleoceanographic, and biological events, has been studied mostly from indirect, often distal, geochemical and biotic evidence. We have upgraded existing geologic mapping in central Panama with more than 2000 fi eld stations, over 40 petrographic analyses, and more than 30 new geochronological and thermo chrono logical analy ses...

The production rate of cosmogenic He in apatite, zircon, kyanite and garnet was obtained by cross-calibration against Be in co-existing quartz in glacial moraine boulders from the Nepalese Himalaya. The boulders have Be ages between 6 and 16 kyr and span elevations from 3200 to 4800 m. In all of these minerals He correlates with Be and is dominantly cosmogenic in origin. After modest correction...

Common naturally occurring orthosilicates can be grouped into those in which the major non-tetrahedral cations are (1) FeII and/or Mg, plus or minus Al (olivine, pyralspite garnets, staurolite); (2) Al (Al2SiO5 polymorphs), or (3) Zr. Feand Mg-bearing orthosilicates have the weakest bonds between non-tetrahedral cations and structural oxygen (M-O bonds), and zircon the strongest, with Al-O bond...

The Chenaillet Ophiolite is one of the best-preserved remnants of the Piemont-Liguria oceanic basin, a branch of the Central Atlantic that opened during the separation of Adria/Africa from Europe. Despite numerous studies of structure, petrology, geochemistry and isotope geochronology, the timing and genesis of various magmatic rocks within the Chenaillet Ophiolite are still controversial. We p...

U-Pb ages obtained on zircons from the investigated migmatites place new constraints on the evolution of the crustal rocks in the Takab area. SHRIMP U-Pb dating of inherited zircon in the melanosome parts of mafic migmatites from the Takab complex gives a discordant age with an upper intercept of 2961±72 Ma (MSWD=0.5) and a lower intercept of 24.2±5.7 Ma (MSWD=1.7). In the concordia diagram, th...