Densification and rheological changes to subducted felsic continental crust in the Sunnfjord area, Norwegian Caledonides: petrogenesis, preservation and implications for scandian collision dynamics

This thesis consists of three principal chapters responding to different questions related to peridotite origin and their behavior in crust. The first chapter deals with geochemistry and petrogenesis of peridotite together with a model of origin of host rock before orogenesis. Chapter is focused on the geochemical, petrogenetic and geochronological investigations of one of the most prominent lower crustal complex in the core of Variscan orogen – the Naměsť Granulite Massif (Bohemian Massif). This chapter is specifically dealing with a new set of whole-rock geochemical data, petrophysical properties of the rocks and (SHRIMP) U-Pb zircon in situ dating of mineral fabrics in granulites. The next chapter deals with combined structural and petrofabric study of Mohelno peridotite and attempt to explain mechanical behavior of peridotite thrust sheet in the frame of polyphase tectonic history of continental root. Our dataset shows mechanical coupling between peridotite sheet and host granulite, pointing to relatively low rheological contrast between upper-mantle and lower-crustal rocks in lower crust conditions. The third chapter is devoted to analysis of development of serpentinisation and AMS fabric in shallow crustal levels during final exhumation stages. An attempt is made to understand magnetic fabric from this highly serpentinized peridotite, its link to the olivine and pyroxene microstructure and tectonic evolution of the whole area. Finally, the last part are general conclusions of so far achieved results combined with some new petrological data and a perspective of large scale geotectonic model of the eastern branch of the Bohemian Massif orogenic lower crust.

Peridotite

Massif

Source

Cite

Citations (0)

Petrological Nature of the Stable Continental Crust

Geophysical monograph (2011)

A. E. Ringwood D. H. Green

A widely accepted geochemical-geophysical model of the stable continental crust consists of a 'granitic'︁ upper layer passing downward into a 'gabbroic'︁ lower layer. This model is examined in the light of recent experimental and petrologic evidence. Under the P-T conditions existing in the lower crust, rocks of gabbroic composition would crystallize dominantly as eclogites under 'dry'︁ conditions and as amphibolites under 'wet'︁ conditions. Accordingly, the concept of a gabbroic lower crust should be abandoned. It is believed that large regions of the lower crust in stable continental areas are essentially dry. In these regions, which have usually undergone a complex orogenic, magmatic, and metamorphic evolution, the lower crust is probably composed of acidic-intermediate rocks in the eclogite facies, i.e. mineralogies of quartz + jadeitic pyroxene + garnet + alkali feldspars. In regions where the crust has undergone a comparatively simple evolution under wet conditions, however, the lower crust might contain large amounts of amphibolite; the structure beneath Lake Superior appears to belong to this category. Possibly it has evolved from a thick pile of iron-rich basaltic lavas which were extruded under submarine conditions and have not undergone subsequent strong deformation or heating above 600°C. As a result the upper crust might now be composed largely of basic rocks in the greenschist facies whereas the lower crust is dominantly composed of amphibolites. This model provides a reasonable match to the inferred geophysical properties of the crust beneath Lake Superior.

Adakite

Metamorphic core complex

Greenschist

Underplating

Pyroxene

10.1029/gm010p0611

Cite

Citations (35)

Crustal underplating in the North American Craton: Evidence from the Petrogenesis of anorthositic rocks in the Middle Proterozoic Midcontinent Rift System

P. W. Weiblen J. D. Miller

Circumstantial evidence for underplating as a significant mechanism for the growth of continental crust continues to accumulate, particularly from considerations of thermal, seismic, and density contraints on the lower crust and upper mantle. Compelling geochemical and petrological arguments were made that in continental flood basalt provinces, the volume of underplated cumulates may be at least as large as the expression of surface lavas. Geological, petrological, and geochemical data are presented on anorthositic rocks in the Duluth Complex, which indicates that underplating processes similar to those postulated by Cox played a significant role in their origin. The interpretations may be applicable to the origin of anorthositic rocks in general.

Underplating

Petrogenesis

Flood basalt

Source

Cite

Citations (1)

Tectonic development from oceanic subduction to continental collision: Geochemical evidence from postcollisional mafic rocks in the Hong'an–Dabie orogens

Gondwana Research (2013)

Li‐Qun Dai Zi‐Fu Zhao Yong‐Fei Zheng

Peridotite

Continental arc

Felsic