CONSTRUCTING CONTINENTAL CRUST: TRACKING PETROGENESIS OF GRANITIC BATHOLITHS OF THE OSLO RIFT, NORWAY

Granitoids compose ~86% of the Earth’s upper crust. Integrating petrographic and geochemical studies of granitic batholiths and their crystal cargoes has the potential to unravel the processes through which granitoid magmatism contributes to upper crustal growth.

The Permo-Carboniferous Oslo Rift (OR) in southeastern Norway contains ~63,000km³ of chemically diverse magmatic rocks, including two batholiths: the Drammen (1,811km³) and the Finnemarka (336km³). These batholiths are predominantly composed of quartz, alkali feldspar, plagioclase, biotite (variably chloritized) and amphibole with minor titanite, apatite, and zircon. The petrogenesis of these granitoid magmas will be investigated using their crystal’s chemical stratigraphy as 1) a tracer of magmatic source and 2) the nature of the magmatic system in which they crystallized (open vs. closed).

Feldspars analyzed to date include sanidine (n=304) and albite (n=155), with relatively minor oligoclase (n=77) and anorthoclase (n=34). Preliminary measurements (n=167) of titanite reveal a strong (n=155) crustal-derived signature (Ti⁴⁺/(Al+Fe³⁺) < 9) and a less prominent (n=12) mantle-derived signature (Ti⁴⁺/(Al+Fe³⁺) > 9). From biotites (n=108), wt.% FeO and MgO abundances are consistent with a mixed mantle-crust derived (3 < wt.% MgO < 13) source (n=74), with significantly fewer mantle-derived (wt.% MgO > 13) biotites (n=34). From amphibole analyses (n=131), wt.% TiO₂ and Al₂O₃ are consistent with crustal-derived (Al₂O₃ < 11) amphibole (n=128) and a lack of mantle-derived (Al₂O₃ > 11) amphibole (n=3). This is also consistent with inferences from Si and Al cation proportions in the sampled amphiboles, where Al < 0.5 is crustal-derived (n=128) and Al > 0.5 is mantle-derived (n=3).

Preliminary data indicates the role of both crustal and mantle-derived sources during the petrogenesis of the OR granitic batholiths. Future analyses will involve bulk rock chemical, Sr-Nd-Pb isotopic, and in-situ trace element characterization of phases sampled to date. Feldspar crystals will also be characterized for their Sr-isotopic signatures. Through a comprehensive geochemical evaluation of these batholiths, and their crystal populations, the magmatic processes involved in the petrogenesis of granitoid rocks will be investigated.