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.

Geology and Ages of Buried Precambrian Basement Rocks, Manitoulin Island, Ontario

1975 ◽  
Vol 12 (7) ◽  
pp. 1175-1189 ◽  
Author(s):  
W. R. Van Schmus ◽  
K. D. Card ◽  
K. L. Harrower
Keyword(s):  
Granitic Rocks ◽  
Precambrian Basement ◽  
Aeromagnetic Data ◽  
Grenville Province ◽  
Metasedimentary Rocks ◽  
Basement Rocks ◽  
Metavolcanic Rocks ◽  
Northern Half ◽  
East End ◽  
Archean Basement

The geology of the buried Precambrian basement under Manitoulin Island in northern Lake Huron, Ontario, has been re-evaluated on the basis of aeromagnetic data, well cuttings, core samples, and rubidium–strontium and uranium–lead geochronologic data on some of the subsurface samples. We conclude that the northern half of the island is underlain in part by Huronian metasedimentary rocks, but that these are absent from the southern part of the island, which is underlain by granitic, gneissic, and metavolcanic rocks. Granitic and gneissic rocks are also present under the northern half of the island.Geochronologic data show that rocks underlying major positive aeromagnetic anomalies are quartz-monzonitic composite plutons which are about 1500 ± 20 m.y. old. Surrounding metasedimentary. gneissic, and granitic rocks are at least 1700 m.y. old. No evidence was found for extrapolation of the pre-Huroman Archean basement beneath Manitoulin Island; if it is present it has been affected by younger metamorphic overprinting.The south west ward extension of the boundary zone between the Grenville Province and rocks to the west can he traced along the east end of Manitoulin Island on the basis of aeromagnetic data.

Petrogenesis of the Carboniferous Ghaleh-Dezh metagranite, Sanandaj–Sirjan zone, Iran: constraints from new zircon U–Pb and 40Ar/39Ar ages and Sr–Nd isotopes

2020 ◽  
Vol 157 (11) ◽  
pp. 1823-1852 ◽  
Author(s):  
Nahid Shabanian ◽  
Ali Reza Davoudian ◽  
Hossein Azizi ◽  
Yoshihiro Asahara ◽  
Franz Neubauer ◽  
...  
Keyword(s):  
Fractional Crystallization ◽  
Crustal Contamination ◽  
Granitic Rocks ◽  
Precambrian Basement ◽  
Early Permian ◽  
Nd Isotopes ◽  
Upper Carboniferous ◽  
Peraluminous Granites ◽  
Sanandaj Sirjan Zone ◽  
Mantle Magma

AbstractThe Ghaleh-Dezh metagranites in the northern Sanandaj–Sirjan Zone (SaSZ) in western Iran are found in a 0.5 km long by 0.3 km wide unit emplaced within the older Precambrian basement. New zircon U–Pb ages confirm that crystallization and emplacement of the protolith of the metagranites occurred at 312 ± 10 Ma and 298 ± 17 Ma in the Upper Carboniferous (Pennsylvanian) – Early Permian, which is consistent with the ages of recently discovered Palaeozoic granites in the northern SaSZ. The studied metagranitic body has been metamorphosed at lower greenschist facies and deformed in ductile–brittle regime due to subsequent reheating events during the Mesozoic. The rocks are metaluminous to slightly peraluminous granites with an A2-type affinity. Initial 87Sr/86Sr ratios and εNd(t) contents vary from 0.7037 to 0.7130 and −0.70 to 0.34, respectively. 143Nd/144Nd(i) values for the granitic rocks are fairly uniform at ∼0.5123. The geochemical and isotopic evidence indicates that these rocks were generated from a mantle magma with crustal contamination and fractional crystallization. Rb–Sr isochron and 40Ar/39Ar K-feldspar ages are 274 Ma and 60–70 Ma, respectively. The older event, c. 270 Ma, was likely related to the opening of Neotethys, whereas the younger ages likely relate to collisional events in the region during the closure of Neotethys.

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>

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.

The geochronology of the granitic rocks along the Bear–Slave Structural Province boundary, northwest Canadian Shield

1977 ◽  
Vol 14 (6) ◽  
pp. 1356-1373 ◽  
Author(s):  
Rosaline Frith ◽  
R. A. Frith ◽  
R. Doig
Keyword(s):  
Age Groups ◽  
Canadian Shield ◽  
Granitic Rocks ◽  
Thermal Event ◽  
Slave Province ◽  
Fault Block ◽  
Granitic Intrusions ◽  
Main Period ◽  
Archean Crust ◽  
Archean Basement

Archean granitic rocks along the southern Bear–Slave boundary fall into three age groups: the oldest are 3000 Ma old intrusive tonalites and granodiorites that form the basement to the Yellowknife Supergroup; the second are syn-volcanic granitic intrusions of ~ 2700 Ma; and the youngest are ~ 2560 Ma granitic and migmatitic diapirs formed in part from supracrustal and granitic rocks. Two Proterozoic thermal events are recognized within the Slave Province. A ~ 2300 Ma event may be related to early rift breakup of the Archean crust and is recorded in Rb–Sr whole-rock and K–Ar mineral systems. A ~ 1970 Ma event was less intense but may be related to further rifting of the Archean and to fault-block depression of the Indin Lake supracrustal basin, the intrusion of a group of granodioritic stocks, and the formation or granitic pegmatite.Within the Bear Province, evidence of a ~ 2700 Ma intrusive event and a ~ 2300 Ma thermal event are preserved in Rb–Sr whole rock systems. Practically all the granitic rocks of the Bear Province, including the Hepburn batholitic rocks, are thought to have been derived wholly or partly from Archean rocks. The main period of Hudsonian deformation and metamorphism was accompanied by a diapiric remobilization of the Archean basement about 1800 Ma ago. Twelve Rb–Sr isochrons, as well as other published geochronologic data from the region, support these conclusions.

U–Pb geochronology in the Trans-Hudson Orogen, northern Saskatchewan, Canada

1987 ◽  
Vol 24 (3) ◽  
pp. 407-424 ◽  
Author(s):  
W. R. Van Schmus ◽  
M. E. Bickford ◽  
J. F. Lewry ◽  
R. Macdonald
Keyword(s):  
Granitic Rocks ◽  
Early Proterozoic ◽  
Tectonic Events ◽  
Northern Saskatchewan ◽  
High Grade Metamorphism ◽  
The One ◽  
Relationship Of ◽  
The Relationship ◽  
Orogenic Events ◽  
Archean Basement

We have obtained U–Pb ages on zircons from volcanic and plutonic units in several lithotectonic domains of the southern Trans-Hudson Orogen in northern Saskatchewan. These data constrain the timing of early Proterozoic orogenic events in the region and enhance our understanding of both the relationships among local domains and the relationship of the Trans-Hudson Orogen to other early Proterozoic orogens in North America.With one exception, all units studied so far yield zircon ages of 1890–1835 Ma, most of which are systematically earlier than previously reported Rb–Sr isochron ages on the same or similar units, suggesting open-system behavior in the Rb–Sr systems. Five metarhyolites, from volcanic sequences in the La Ronge domain, Glennie domain, and Hanson Lake block, give ages ranging from 1888 to 1876 Ma. Most of the plutons we dated, ranging from gneissic syntectonic tonalites and granodiorites to less-deformed late intrusions such as the Wathaman batholith and other smaller bodies, yield ages of 1870–1850 Ma, apparently constraining peak plutonic activity to about 1860 ± 10 Ma ago. The youngest unit found is a small discordant pluton with an age of 1836 ± 7 Ma. The concordance of ages of volcanics on the one hand and of plutons on the other suggests that domainal distinctions are mainly lithotectonic rather than temporal.Zircons from the Sahli charnockitic granite in the Hanson Lake block yield equivocal results. Discordia upper and lower intercepts for the Sahli granite suggest that granitic rocks at least 2500 Ma old were subjected to high-grade metamorphism about 1800–1900 Ma ago, with substantial resetting of zircons. Reworked Archean basement is thus present in this domain, supporting previously reported Rb–Sr isochron data from the Sahli granite. No other indications of Archean basement in the Trans-Hudson Orogen are documented, although one sample from the adjacent Peter Lake domain shows that it consists of Archean continental crust.Zircon ages in the range 1890–1835 Ma from this part of the Trans-Hudson Orogen are similar to those obtained from igneous units of the Penokean and Wopmay orogens, in North America, and from the Svecofennian Orogen, suggesting essential synchroneity of igneous and tectonic events in these four major orogens during major Proterozoic continental assembly.

scholarly journals Settlement and Society in Early Imperial Etruria

2006 ◽  
Vol 96 ◽  
pp. 88-123 ◽  
Author(s):  
Robert Witcher
Keyword(s):  
Survey Data ◽  
Methodological Issues ◽  
Regional Survey ◽  
Regional Patterns ◽  
Imperial Period ◽  
Structural Connections ◽  
Similarities And Differences ◽  
The Impact ◽  
Survey Comparison

This paper compares the early imperial period results from thirty surveys in and around regio VII Etruria in order to identify similarities and differences of settlement, population, and economy. Three sub-regional patterns are defined: the suburbium, coastal Etruria, and inland Etruria. Consideration of methodological issues of survey comparison suggests the problem is real, but not insuperable. A range of interpretative models is discussed with particular reference to the impact of the Urbs on economic, agricultural, and social developments. The structural connections between these sub-regions are emphasized, particularly the organization of labour, demography, and agricultural strategies. The results reveal varied responses to Roman control, leading to more not less diversity. More generally, the results underline the value of comparing regional survey data.

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