HR: 1340h
AN: V13E-0581    [Abstracts]
TI: Sheeting, Mingling, and Up Direction?, Hortavaer Igneous Complex, North-central Norway
AU: * McCulloch, L E
EM: lindy.mcculloch@ttu.edu
AF: Texas Tech Univ., Dept. of Geosciences, Lubbock, TX 79409-1053 United States
AU: Barnes, C G
EM: cal.barnes@ttu.edu
AF: Texas Tech Univ., Dept. of Geosciences, Lubbock, TX 79409-1053 United States
AU: Prestvik, T
EM: tore.prestvik@geo.ntnu.no
AF: Norwegian Institute of Science and Technology, Dept. of Geology, Trondheim, N-7491 Norway
AB: The c. 456 Ma Hortavaer igneous complex intrudes quartzofeldspathic gneiss, quartzite, marble and calc-silicate gneiss of the Helgeland Nappe Complex of the Uppermost Allochthon. The intrusive rocks may be divided into at least three distinct zones, each of which contains screens/xenoliths of the host rocks. The western "syenitic zone" is predominantly fine- to very coarse-grained syenite. Grain size is variable on the outcrop scale, primarily due to intrusion and mingling of fine-grained syenite in coarser grained syenite. The eastern "dioritic zone" consists of hundreds of mafic sheet-like intrusions (gabbro to monzodiorite), metasomatized calc-silicate rocks (now melanocratic syenite and monzonite), monzonite, and syenite. Cross-cutting relationships, particularly back-veining of syenite in mafic bodies, indicate that syenitic magmas were present throughout emplacement of the mafic magmas. The boundary between syenitic and dioritic zones consists of parallel to subparallel syenite and diorite sheets from cm to c. 10 m wide. The sheets strike N to NE and dip W to NW between 40° and 60°. In this "sheeted zone", possible hybrid rocks are identified by their intermediate color index, their occurrence as enclaves within syenite and diorite, and locally by the presence of syenitic and dioritic enclaves in intermediate rocks. Contacts between sheeted zone syenite and diorite are asymmetric. The western contacts of many diorite sheets are chilled against syenite and locally show flame- and load cast- like structures. In contrast, the eastern contacts range from planar to wispy or crenulate, and dioritic enclaves are common. If this asymmetry is used as a geopetal indicator, then original up direction was to the east and the Hortavaer complex is now overturned. The nearby Leka ophiolite underwent block rotation of about 90° to the east; similar, larger eastward block rotation of the Hortavaer complex would result in overturning of the sheets. Reconstruction of the complex assuming original horizontal orientation of the sheeted zone indicates an early, 1500 m-thick basal syenitic magma sequentially intruded by additional syenite. This was followed by a transition to overlying dioritic magmas to form the sheeted zone (c. 400 m thick) and then by a predominance of dioritic magmas to form the dioritic zone (>1700 m thick). Throughout this history, syenitic magma was present, as shown by mutual cross-cutting relationships between syenite and diorite throughout the complex.
DE: 1036 Magma chamber processes (3618)
DE: 1090 Field relationships (3690, 8486)
DE: 8439 Physics and chemistry of magma bodies
DE: 9335 Europe
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005