Using whole rock geochemistry to locate the source of igneous erratics from drumlins on the Atlantic coast of Nova Scotia

Whole rock geochemistry has been determined for erratics from the surface Lawrencetown Till in two drumlins on the Atlantic coast of Nova Scotia. The geochemical data, particularly trace element composition, allow source rocks to be identified more precisely than by traditional petrographic microscopy. A drumlin near Lunenburg contains erratics of Neoproterozoic arc-related plutonic rocks and basalts that outcrop only in a small area near Parrsboro in the Cobequid Highlands. Some associated erratics of granite precisely match a small late Devonian pluton in the same area and other granites and rhyolite could also be derived from late Devonian rocks in the same area. The type section of the Lawrencetown Till contains different Neoproterozoic plutonic rocks of a type found in the eastern Cobequid Highlands, together with a distinctive late Devonian granite with sodic amphiboles that is restricted to a small area near the Debert River. Both these source areas indicate a southward flow of ice (Escuminac Phase) depositing the Lawrencetown Till, in contrast to earlier southeastward-flowing ice (Caledonia Phase) which deposited the Hartlen Till that forms the core of the drumlin. Drumlins in Nova Scotia are complex palimpsest landforms that were formed by till accretion, and evolved in surface form throughout multiple flow events.     

Zoned Pyroxenes from Shoshonite Lavas of Lesbos, Greece: Inferences concerning Shoshonite Petrogenesis

Samples from several stratigraphic levels in the Miocene volcanicsuccession on Lesbos, Greece, contain complexly zoned clinopyroxeneand feldspar phenocrysts. Electron microprobe analyses havebeen made of zoned clinopyroxenes and feldspars, and their inclusions;and phenocryst mineralogy is compared with host-rock chemicalcomposition. The commonest pyroxene zoning sequence is coresof greenish salite or augite mantled by diopside which, in turn,may be rimmed by augite, with each type of pyroxene in opticaland chemical discontinuity with the next. None of the pyroxeneshas high pressure characteristics. The Ts content of phenocrystcores is correlated with the K-content of the host rock; andthe most diopsidic overgrowths also occur in the most K-richrocks. Augite rims occur only in Al-rich rocks. Most of the features of the pyroxenes can be accounted for bya hypothetical fractional crystallization scheme. However, twoobservations suggest that mixing of small magma batches mayalso be important. The earliest crystallized diopsides thatrim salite are rich in Cr₂O₃, suggesting precipitation froman unfractionated magma. Anorthoclase (implying advanced crystallizationof feldspars) occurs as cores to feldspar phenocrysts and asinclusions in clinopyroxene cores in several samples.     

Granitoid rocks of Naxos,Greece: regional geology and petrology

Granitic gneiss in a Miocene extensional core complex on Naxos locally preserves primary igneous textures. On an outcrop scale, these include mafic enclaves; in thin section, feldspar phenocrysts contain unoriented accessory mineral inclusions. The gneiss is interpreted as having a Hercynian granite protolith. Contrary to previous accounts, migmatites are rare in the gneiss. The granite is geochemically similar to post-collisional extension-related granites and differs from the predominant granodiorites found in the Hercynian basement of northwestern Greece. An I-type hornblende–biotite granite pluton was emplaced during Miocene extension in western Naxos. It is a typical subduction-related pluton emplaced under conditions of back-arc extension. The pluton is cut by later leucogranite that geochemically resembles the granite dykes that cut the migmatites. In northern Naxos, minor leucogranite intrusions are of two geochemical types. One is everywhere deformed and geochemically resembles the leucogranite that cuts the Western pluton. The other is variably deformed and new geochronology shows that it has an age of 10 Ma. © 1997 John Wiley & Sons, Ltd.