Notes: [Auszug] The ‘Late Heavy Bombardment’ was a phase in the impact history of the Moon that occurred 3.8–4.0 Gyr ago, when the lunar basins with known dates were formed. But no record of this event has yet been reported from the few surviving rocks of this age on the Earth. Here we ...

Notes: [Auszug] Samarium–neodymium isotope data for tectonically interleaved fragments of lithospheric mantle and meta-komatiite from the North Atlantic craton provide the first direct record of mantle differentiation before 3r800 Myr ago. The results confirm the magnitude of light-rare-earth-element ...

Notes: Abstract The 3,622±72 m.y. Uivak gneisses of northern Labrador consist of two major rock suites: regionally developed layered granodioritic gneisses (Uivak I suite) interpreted as derived from earlier tonalitic igneous parents by the massive introduction of potassium and rubidium, and a less extensive group of iron-rich porphyritic granodiorites and ferro-diorites (Uivak II suite) emplaced after at least one major period of deformation and migmatisation had affected the Uivak I gneisses. Samples from both suites fall on the 3,622 m.y. Rb/Sr whole-rock isochron. The age is provisionally interpreted as that of Rb metasomatism and homogenisation affecting both suites. It is suggested that the low initial ratio of 0.7014±0.0008 obtained from both suites need not be diagnostic of a short crustal residence if the evidence for massive addition of Rb to this level of the crust at 3,622 m.y. is accepted.

Notes: Abstract A petrogenetic model is developed to explain the evolution and geochemical character of granitic rocks in early Archean (pre 3.6 Gyr) continental crust taking into account the following important geological constraints, viz.: 1. High geothermal gradients (probably in excess of 90 ° C/km) and resulting widespread granulite facies metamorphism even at relatively shallow depths 2. The fractionation of certain major and trace elements under granulite facies conditions 3. The composition and geochemical behaviour of fluids which emanate from or pass through terrains undergoing granulite facies metamorphism viz. carbonic fluids containing significant amounts of SO2 and halogens. In this model tonalitic and trondhjemitic intrusives are regarded as being derived dominantly by partial melting of mafic granulite. The ubiquitous potassic granites, which typical post-date sodic plutonic activity are interpreted to be anatectic melts generated under granulite or amphibolite facies conditions from the previously formed ‘plagiogranites’. The presence of a postulated granulite facies source area for Archean tonalitic rocks, and the geochemical character of fluids which accompany metamorphism under such conditions explains the HREE geochemistry of these suites and casts doubt on the validity of applying currently used trace element fractional melting or crystallization models to these terrains. Similarly it suggests that petrogenetic interpretations based on Sr and Pb isotopic systems must be reevaluated because of the extreme mobility of both parent and daughter elements under granulite facies conditions.

Notes: Abstract Anorogenic granites of middle to late Proterozoic age in the Davis Inlet — Flowers Bay area of Labrador are subdivided on the basis of petrology and geochemistry into three coeval suites. Two of these are high-temperature anhydrous hypersolvus granites: a peralkaline aegirine-sodic-calcic to sodic amphibole-bearing suite and a non-alkaline fayalite-pyroxene-bearing suite. The third is a group of non-alkaline subsolvus hornblende-biotite-bearing granites. Associated with the hypersolvus peralkaline suite is a group of genetically related syenites and quartz syenites. The granites cut ca. 3,000 Ma old Archaean gneisses as well as Elsonian layered basic intrusions of the Nain Complex. One of these, a crudely layered mass which ranges in composition from gabbro to diorite and monzonite, appears to be related to the syenites. The peralkaline granites and some of the syenites are extremely enriched in the high field-strength elements such as Y, Zr, Nd, as well as Rb, Ga and Zn, and have low abundances of Ba, Sr and most of the transition elements. In contrast, the non-alkaline hypersolvus and subsolvus granites do not show the same degree of enrichment. Concentration of the highly charged cations in the peralkaline suite is believed to be the result of halogen-rich fluid activity during fractionation of the magma. The sodic evolution trend in the peralkaline suite is reflected mineralogically by the development of aegirine and aegirine-hedenbergite solid solutions, and by a spectacular amphibole compositional range from katophorite through winchite, richterite, riebeckite to arfvedsonite and ferro eckermannite. Accessory phases which are ubiquitous in these rocks include aenigmatite, astrophyllite, fluorite, monazite and zircon. The non-alkaline hypersolvus granites typically contain iron-rich phases such as fayalite, eulite, ferrosilite-hedenbergite, and annite rich biotite. In the subsolvus granites, amphiboles range in composition from edenite through common hornblende to actinolite and also coexist with annite-rich biotite. Whole-rock and mineral isotopic data for the different suites yield isochrons that are within error of ca. 1,260 Ma, but they have variable initial 87Sr/86Sr ratios. The initial 87Sr/86Sr of the syenites and peralkaline granites (0.7076±11) is significantly lower than the initial 87Sr/86Sr of the subsolvus granites (0.7138±22). These isotopic data provide further confirmation of the importance of a late Elsonian alkaline event in Labrador which can be correlated with Gardar igneous activity in south Greenland. The petrogenesis of the peralkaline suite is interpreted to reflect the effects of fractionation of anhydrous phases from mantle derived basic magma which was contaminated during ascent by radiogenic partial melts of crustal derivation. The non-alkaline hypersolvus and subsolvus granites are interpreted as crustal melts which formed under conditions of variable $$P_{{\text{H}}_{\text{2}} {\text{O}}} $$ in response to the same thermal event, and which subsequently experienced feldspar fractionation during crystallization.