METAMORPHOSED PLUTONIC ROCKS OF THE SCANDIAN UPPER ALLOCHTHON, COASTAL MID NORWAY: INTRUSIVE ROOTS OF AN ORDOVICIAN- EARLIEST SILURIAN CALC-ALKALINE ARC

The Støren Nappe of the Upper Allochthon of Mid-Norway has four major parts. The Støren Group (1) consists of overwhelmingly basaltic Late Cambrian-Early Ordovician oceanic ophiolite complexes. These were obducted onto the peri-continental Gula Group (2) in the Iapetus Ocean. On these an Ordovician-Silurian arc was built, beginning in the back-arc region with strata of the Hovin Group (3) that has fossils of the Laurentian Toquima-Table Head fauna. NW of (3) and intruding it and the ophiolites are plutonic rocks (4) of an Ordovician - Early Silurian magmatic arc with U-Pb zircon ages of 482-442 Ma. Thrusting of (1-4) onto Baltica occurred in the Silurian-Devonian Scandian Orogeny. Much work has been done on (1), (2), (3). Here we report 153 new analyses of samples of (4) from near the coast SW, W, and NW of Trondheim. They are apparently derived from the plutonic arc roots, with rare recognizable volcanics. SE of the Høybakken brittle detachment most are strongly deformed gneisses, but to the NW on the islands of Hitra and Smøla they are almost undeformed. They span a continuous composition range of 45-77% SiO₂, and from hornblendite to granite, with no bimodal tendency as is seen in volcanic rocks in the Støren Group (1). Most mafic rocks are tholeiitic, some calc-alkaline, one alkaline, but rocks with >57% SiO₂ are overwhelmingly calc-alkaline. There are two distinct geochemical domains: In the Lensvik Synform, and its extension along strike to the NE, these rocks are dominated by tonalitic gneisses with flat, or slightly REE-enriched or -depleted patterns that closely resemble oceanic ‘plagiogranites; weak to strong negative Nb-Ta anomalies indicating arc affinity, resembling anomalies in Støren Group mafic volcanics (1); and small negative to no Eu anomalies. The larger domain, across strike to the NW, is dominated by tonalites with strongly enriched LREE and variably depleted HREE patterns, suggesting variable residual garnet and plagioclase in the magma source regions; uniformly large negative Nb-Ta anomalies; and small Eu anomalies (~half), no Eu anomalies (~half), or positive Eu anomalies (a few). Excluding cumulates, mafic rocks range from LREE-depleted to strongly LREE-enriched. Metamorphic mineral assemblages indicate peak epidote- amphibolite-facies metamorphism, with no evidence for partial melting.