The Precambrian problem in younger orogenic zones: An example from Japan

1968 ◽  
Vol 5 (3) ◽  
pp. 643-648 ◽  
Author(s):  
Tatsuro Matsumoto ◽  
Masaru Yamaguchi ◽  
Takeru Yanagi ◽  
Susumu Matsushita ◽  
Ichikazu Hayase ◽  
...  
Keyword(s):  
Radiometric Dating ◽  
Granitic Rocks ◽  
Northwestern Part ◽  
Precambrian Basement ◽  
Early Paleozoic ◽  
Central Japan ◽  
Field Evidence ◽  
Orogenic Zone

We have examined some of the presumed Precambrian basement metamorphic and granitic rocks in Japan, through radiometric dating as well as on field evidence, and have found that mineral ages of about 175 to 250 m.y. are abundant in the Hida area, northwestern part of central Japan, that a number of thrust rocks in southwestern Japan show ages of 400 to 450 m.y., and that the oldest of the measured samples is about or somewhat over 500 m.y. Little evidence is available to support a view that the Pre-Sinian rocks, if ever existent, have remained unaltered under such a polycyclic orogenic zone as that represented by Japan, although remnants of the youngest Precambrian to Early Paleozoic cycle can be detected.

Isotopic data bearing on the origin of Mesozoic and Tertiary granitic rocks in the western United States

1984 ◽  
Vol 310 (1514) ◽  
pp. 743-753 ◽  
Keyword(s):  
Granitic Rocks ◽  
Precambrian Basement ◽  
Regional Survey ◽  
Regional Patterns ◽  
Peraluminous Granite ◽  
Isotopic Compositions ◽  
Geographic Coverage ◽  
Magma Flux ◽  
Archean Basement ◽  
Mantle Magma

A regional survey of initial Nd and Sr isotopic compositions has been done on Mesozoic and Tertiary granitic rocks from a 500 000 km 2 area in California, Nevada, Utah, Arizona, and Colorado. The plutons, which range in composition from quartz diorite to monzogranite, are intruded into accreted oceanic geosynclmal terrains in the west and north and into Precambrian basement in the east. Broad geographic coverage allows the data to be interpreted in the context of the regional pre-Mesozoic crustal structure. Initial Nd isotopic compositions exhibit a huge range, encompassing values typical of oceanic magmatic arcs and Archean basement. The sources of the magmas can be inferred from the systematic geographic variability of Nd isotopic compositions. The plutons in the accreted terrains represent mantle-derived magma that assimilated crust while differentiating at deep levels. Those emplaced into Precambrian basement are mainly derived from the crust. The regional patterns can be understood in terms of: (1) the flux of mantle magma entering the crust; (2) crustal thickness; and (3) crustal age. The mantle magma flux apparently decreased inland; in the main batholith belts purely crustal granitic rocks are not observed because the flux was too large. Inland, crustal granite is common because mantle magma was scarce and the crust was thick, and hot enough to melt. The values of peraluminous granite formed by melting of the Precambrian basement depend on the age of the local basement source.

Collisional Orogeny in the Scandinavian Caledonides (COSC): Some preliminary results from drilling of the 2.276 km deep COSC-2 borehole, central Sweden

2021 ◽  
Author(s):  
Christopher Juhlin ◽  
Bjarne Almqvist ◽  
Mark Anderson ◽  
Mark Dopson ◽  
Iwona Klonowska ◽  
...  
Keyword(s):  
High Amplitude ◽  
Granitic Rocks ◽  
Magnetic Data ◽  
Precambrian Basement ◽  
Tectonic Model ◽  
Core Recovery ◽  
Alum Shale ◽  
Borehole Seismic ◽  
Seismic Reflectors ◽  
Seismic Measurements

<p>COSC investigations and drilling activities are focused in the Åre-Mörsil area (Sweden) of central Scandinavia. COSC-2 was drilled with nearly 100% core recovery in 2020 to 2.276 km depth with drilling ongoing from mid-April to early August. Drilling targets for COSC-2 included (1) the highly conductive Alum shale, (2) the Caledonian décollement, the major detachment that separates the Caledonian allochthons from the autochthonous basement of the Fennoscandian Shield, and (3) the strong seismic reflectors in the Precambrian basement.</p><p>Combined seismic, magnetotelluric (MT) and magnetic data were used to site the COSC-2 borehole about 20 km east-southeast of COSC-1. Based on these data it was predicted that the uppermost, tectonic occurrence of Cambrian Alum shale would be penetrated at about 800 m, the main décollement in Alum shale at its stratigraphic level at about 1200 m and the uppermost high amplitude basement reflector at about 1600 m. Paleozoic turbidites and greywackes were expected to be drilled down to 800 m depth. Below this depth, Ordovician limestone and shale with imbricates of Alum shale were interpreted to be present. Directly below the main décollement, magnetite rich Precambrian basement was expected to be encountered with a composition similar to that of magnetic granitic rocks found east of the Caledonian Front. The actual depths of the main contacts turned out to agree very well with the predictions based on the geophysical data. However, the geology below the uppermost occurrence of Alum shale is quite different from the expected model. Alum shale was only clearly encountered as a highly deformed, about 30 m thick unit, starting at about 790 m. Between about 820 and 1200 m, preliminary interpretations are that the rocks mainly consist of Neo-Proterozoic to Early Cambrian tuffs. Further below, Precambrian porphyries are present. The high amplitude reflections within the Precambrian sequence appear to be generated by dolerite sheets with the uppermost top penetrated at about 1600 m. Several deformed sheets of dolerite may be present down to about 1930 m. Below this depth the rocks are again porphyries.</p><p>A preliminary conclusion concerning the tectonic model is that the main décollement is at about 800 m and not at 1200 m. Also the thickness of the lowermost Cambrian/uppermost Neoproterozoic sediments on top of the basement is much greater than expected (hundreds of meters instead of tens of meters) and likely to have been thickened tectonically. Detailed studies are required to assess the actual importance of the “main décollement” and the degree, type and age of deformation in its footwall. We can also conclude that the Precambrian basement is very similar to the Dala porphyries succession that are typically present farther south.</p><p>An extensive set of downhole logging data was acquired directly after drilling. Borehole seismic measurements in 2021 will help to define and correlate seismic boundaries with lithology and structures in the core. Unfortunately, work for describing the geology of the drill core in detail is still on hold due to Covid-19.</p>

Provenance and tectonic implications of Palaeoproterozoic (c. 1740 Ma) quartz porphyry clasts in the basal Old Red Sandstone (Lilljeborgfjellet Conglomerate Formation) of northwestern Svalbard's Caledonides

1998 ◽  
Vol 135 (6) ◽  
pp. 755-768 ◽  
Author(s):  
F. J. HELLMAN ◽  
D. G. GEE ◽  
T. GJELSVIK ◽  
A. M. TEBENKOV
Keyword(s):  
Amphibolite Facies ◽  
Granitic Rocks ◽  
Maximum Diameter ◽  
Northwestern Part ◽  
High Grade ◽  
Quartz Porphyry ◽  
Red Sandstone ◽  
Facies Metamorphism ◽  
Single Zircon ◽  
Red Bay

The Lilljeborgfjellet Conglomerate Formation composes the lower part of the alluvial Siktefjellet Group of northwestern Spitsbergen's Old Red Sandstone succession. Siktefjellet strata are of late Silurian or early Devonian age, but lack precise age-diagnostic fossils. They are unconformably overlain by conglomerates and sandstones of the Red Bay Group, which contain a well established fish fauna of Lochkovian age. The Lilljeborgfjellet Conglomerate rests with a major unconformity on high-grade (with eclogites) schists and gneisses, with associated corona gabbros and granitic gneisses. Previous isotope-age studies have shown that these igneous rocks yield U/Pb ages of c. 950 Ma, and that the eclogite facies metamorphism may be of Caledonian or late Neoproterozoic age. The high P/high T rocks are intercalated with and overlain by schists affected only by Caledonian amphibolite facies metamorphism, recorded by 40Ar/39Ar and Rb/Sr cooling ages of 400–430 Ma.In the Lochkovian Red Bay Group of the Raudfjorden Graben, two horizons of tuffites occur, interbedded with sandstones. New studies of eight zircons from these volcanic rocks have provided single-zircon lead-evaporation ages of c. 950 and c. 1350 Ma; one yielded 440 Ma. All these zircons are probably derived from the underlying basement rocks, the ages being significantly older than the Devonian host strata (c. 410 Ma).The clasts in the Lilljeborgfjellet Conglomerate are generally angular to subrounded and derived locally from the underlying high-grade metamorphic complex. A subordinate (usually less than 1%, but up to about 10%) component of the clasts is a quartz porphyry that is not known in the exposed bedrock anywhere in northwestern Spitsbergen. The quartz porphyries are better rounded than the other clasts; however, the maximum diameter reaches 1.5 metres, indicating that transport distances are unlikely to have exceeded a few kilometres. Three quartz porphyry boulders have been dated by the single-zircon lead-evaporation method and shown to be of Palaeoproterozoic age, yielding ages of 1735±4, 1736±5 and 1739±5 Ma that have not previously been detected in the northwestern part of Svalbard's Caledonides.The quartz porphyry clasts show no evidence of the widespread high-grade tectonothermal activity of Mesoproterozoic and early Palaeozoic age that influenced northwestern Spitsbergen. It is therefore concluded that the most probable source of these clasts lies to the east in the unexposed basement beneath the Old Red Sandstones of the Andrèeland–Dicksonland Graben. The Lilljeborgfjellet quartz porphyry clasts are closely similar in age to the granitic rocks of Ny Friesland. Whereas the latter were subject to Caledonian high amphibolite facies metamorphism, the quartz porphyry clasts have only been affected by a low greenschist facies overprint. Nevertheless, the similarity in age suggests an affinity to Ny Friesland and it is proposed here that the Breibogen–Bockfjorden Fault defines the most important boundary between Svalbard's Caledonian terranes.

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