HR: 1340h
AN: V13E-0598    [Abstracts]
TI: Crustal melting in the Helgeland Nappe Complex, central Norway
AU: * Yoshinobu, A
EM: aaron.yoshinobu@ttu.edu
AF: Dept. of Geosciences, Texas Tech University, Lubbock, TX 79409 United States
AU: Reid, K
EM:
AF: Dept. of Geosciences, Texas Tech University, Lubbock, TX 79409 United States
AU: Barnes, C
EM: cal.barnes@ttu.edu
AF: Dept. of Geosciences, Texas Tech University, Lubbock, TX 79409 United States
AU: Allen, C M
EM:
AF: RSES, Australia National University, Canberra, 0200 Australia
AB: We present new geochronological and structural data and a working synopsis for the evolution of the Helgeland Nappe Complex (HNC), the uppermost nappe sequence within the Caledonian Orogen, central Norway. The HNC consists of at least 4 distinct east-dipping nappes (W to E): Sauren-Torghatten (S-T), Lower, Middle, and Upper. The nappes are bounded by shear zones with complex kinematics. In at least two cases, initial thrust-sense shear is overprinted by extensional deformation. Basal parts of the S-T and Middle Nappes are characterized by incomplete ophiolitic slivers overlain by low- to medium-grade pelites, psammites, conglomerates, and marbles (Vietti et al., this meeting). The Lower and Upper Nappes are characterized by quartzo-feldspathic gneisses, migmatitic gneisses, and marble. New LA-ICPMS U-Pb zircon ages from migmatitic rocks of the upper nappe are 480.6 ±3.8 Ma for a diatexite and 480.2 +/- 3.6 Ma for a melanosome within a stromatic migmatite. These ages are identical to SHRIMP U-Pb (zircon) ages from the Lower Nappe interpreted by Yoshinobu et al. (2002) to date crustal melting. The HNC was intruded by plutons of the Bindal Batholith from ~480-430 Ma. The oldest plutons (479-466 Ma) are crustally-derived peraluminous granites; they intrude the S-T and Lower Nappes. The largest of these, the 470 Ma Vega pluton, was emplaced into rocks similar to the S-T Nappe during a regional migmatization/deformation event (D3; Anderson et al., Marko et al., this meeting). Post-450 Ma plutons intrude all nappes. The oldest post-450 Ma plutons were emplaced at pressures as high as 700 MPa and exhumed to 400 MPa during crystallization at 447 Ma. At least locally, they crosscut nappe-bounding shear zones but some were apparently deformed along exhumation-related shear zones. No quantitative P-T-t estimates are available for the Middle Nappe. The Upper Nappe migmatites contain GASP assemblages yielding pressures of 500 MPa. These migmatitic rocks are intruded by tonalitic intrusions at 447.6±2.8, 431.9±3.5, and 424.7±5.6 Ma. A growing body of evidence indicates that the Ordovician structural, metamorphic, and magmatic history of the HNC has affinities with the Laurentian Taconic Orogeny, rather than with an early phase of the Baltica-Laurentia (Caledonian) collision. These new data indicate that either a) a protracted (480-470 Ma), regionally extensive, thermal and structural event caused crustal melting and produced large crustally-derived granitic plutons in the S-T, Lower, and Upper Nappes, but not the Middle Nappe, or b) contemporaneous but spatially distinct regional crustal melting events that affected all three nappes.
DE: 3642 Intrusive structures and rocks
DE: 3651 Thermobarometry
DE: 8108 Continental tectonics: compressional
DE: 8145 Physics of magma and magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005