The general geology and geochemistry of the granitoid rocks of the northern Gander Lake Belt, Newfoundland

摘要

The granitoid rocks of the northern Gander Lake Belt form three distinct groups, viz. quartz diorites, leucocratic granodiorites, and megacrystic biotite granites roughly in the areal proportions of 3:22:75. The study area is divisible into four distinct N-S trending units which are from west to east, 1. The Davidsville Group, 2. the metasedimentary terrain, 3. the gneissic terrain, and 4. the western Avalon Platform. -- The Davidsville Group is composed of essentially non-metamorphosed Middle Ordovician shales, greywackes, and mafic volcanic rocks, which have been intruded by hornblende-biotite quartz diorites. The plutons show a zonation from hornblende quartz diorite at the margins to biotite quartz diorite in the core. The quartz diorite plutons are mineralogically similar which, along with their extensive and similar contact metamorphic aureoles, suggest that they may be connected at a shallow depth. -- The metasedimentary terrain unconformably underlies the Davidsville Group, and is composed of a monotonous sequence of low greenschist facies metasediments. The metasediments have been syntectonically intruded by muscovite-biotite-garnetiferous leucocratic granodiorites which contain an abundance of pegmatites. To the southeast of Gander, the terrain has been post-tectonically intruded by the Gander Lake pluton which is similar in composition to the megacrystic biotite granites of the gneissic terrain. -- The gneissic terrain is underlain by high grade gneisses, migmatites, and schists, which have been intruded by large plutons of megacrystic biotite granite, and locally by small bodies of leucocratic granodiorite. The megacrystic biotite granite is characterised by its very coarse grain size, and large microcline megacrysts. Locally the megacrystic granites have been intensely mylonitised. The leucogranites are similar to those of the metasedimentary terrain. -- The Precambrian Avalon Platform sediments and low grade schists of the study area have been intruded by megacrystic biotite granite similar to that of the gneissic terrain. -- The mineralogy of the various groups of granitoid rocks is generally reflected in their chemistry. The quartz diorites are the most mafic in composition with a low content of K and Rb, a high content of Ca, Fe, and Sr, and a high K/Rb ratio. The leucocratic granodiorites are the most salic in composition with a mean silica content of almost 72%, a high content of K and Rb, and a low content of Ca, Sr, Mg, and Fe, and a low K/Rb ratio. The megacrystic biotite granites are generally intermediate in composition between the quartz diorites and the leucocratic granodiorites but have the highest content of K and F. The Deadman's Bay pluton has an initial Sr⁸⁷/⁸⁶ ratio of 0.704. K shows a general increase from northwest to southeast across the study area. -- The quartz diorites are similar in geological setting, petrography, and chemistry to the granitoids of island arcs and are interpreted as having a similar origin. The enrichment of the leucocratic granodiorites in lithophile elements, their syntectonic origin, lack of associated basic igneous rocks, and approximately eutectic composition suggests that they were formed by anatexis of continental crust. The origin of the megacrystic biotite granites is especially problematical in that they have a very high K content and a very low initial Sr⁸⁷/⁸⁶ ratio. Neither anatexis of continental crust nor partial melting of oceanic crust can account for these two features; some combination of both processes is possible but present evidence does not allow selection of a particular mode of origin for these rocks. -- The abundance of pegmatites in the Middle Ridge pluton and the known occurrence of beryl associated with these pegmatites may justify exploration for economic concentrations of Be and associated elements. The occurrence of fluorite and locally high fluorine concentrations in stream water of the North Pond area could be re-examined to ascertain the fluorite potential. With the known correlation between tin and fluorine of other mineralized areas the megacrystic biotite granites may also have potential as sources of tin.