Mafic Intrusive Bodies Research Articles

U-Pb zircon geochronology and Sm-Nd-Pb isotopic constraint for Precambrian plutonic rocks in the northeastern part of Ryeongnam massif, Korea

The Ryeongnam massif is composed of Precambrian gneisses, Paleozoic and Mesozoic sedimentary rocks and extensive Triassic-Jurassic plutonic rocks of felsic to mafic composition. In the northeast Ryeongnam massif, the oldest rocks belong to the Sobaegsan gneiss complex, which is composed of orthogneisses, paragneisses and mafic plutonic rocks. U-Pb zircon ages for the felsic and mafic intrusive bodies within the Sobaegsan gneiss complex are: the Icheon granite gneiss, 2357 ± 43 and 2342 ± 47 Ma; the Buncheon granite gneiss, 1963 ± 5 Ma; the Pyeonghae granite gneiss, 1936 ± 21 Ma; the Ogbang amphibolite, 1918 ± 10 Ma; the Imwon leucogranite gneiss, 1826 ± 20 Ma. The Hyeondong biotite schist, which is intruded by the Buncheon granite gneiss and the Ogbang amphibolite, yielded an age of 2271 ± 44 Ma. The Nd-Sm-Pb isotopic data indicate that the felsic plutonic rocks are derived from an older Archean crust. The Nd TDM ages are Archean, and the eNd values are negative for the felsic rocks and positive for the amphibolite. Common Pb isotope compositions also indicate a crustal source for the felsic intrusives. The U-Pb ages of Precambrian rocks of the Ryeongnam massifs are similar to those in the Gyeonggi massif, and may have a similar crustal evolutionary history. The Precambrian rocks of South Korea could be related either to the North China block or to the South China block, as the isotope ages and patterns are not unique to either block. Similarly, a geologic correlation with Japan, although possible, is tenuous at present.

Petrogenesis of an 800 m lava sequence in eastern Uruguay: insights into magma chamber processes beneath the Paraná flood basalt province

Major and trace element along with representative Sr, Nd and Pb isotope data are presented for drill core samples which intersect an 800 m lava pile in eastern Uruguay. The lavas form part of the Paraná flood basalt province, are low-Ti in composition but distinct from the low-Ti Gramado magma type, and have been termed the Treinte Y Trés magma type. The lava pile overlies a large positive gravity anomaly inferred to reflect an east–west trending, mid-crustal mafic intrusive body with a calculated volume of 35,000 km 3. Smooth up-section compositional variations in the basalts are interpreted to record magma evolution within this mid-crustal magma chamber. 87Sr/ 86Sr and 206Pb/ 204Pb increase throughout the sequence yet Mg♯ remains relatively constant in the lower 200 m of the sequence, suggesting a role for magma chamber recharge. Above this the lavas show a regular, up-section decrease in Mg♯ coupled with increasing 87Sr/ 86Sr and 206Pb/ 204Pb and this is interpreted to reflect crystal fractionation combined with crustal contamination. The data provide further evidence that contamination of flood basalt magmas in crustal magma chambers is a common phenomenon and calculations suggest that the amount of crustal addition may be as high as 60–70%. Nevertheless, the effects of this crustal contamination do not appear able to account for the discrepancy between key incompatible trace element ratios and isotope ratios of the lavas and those of any putative mantle plume. In fact, La/Ta decreases with decreasing Mg♯ and increasing 87Sr/ 86Sr indicating that the effects of crustal contamination were actually to reduce La/Ta and implying that the parental magmas had very high La/Ta (∼90). These constraints are clearly inconsistent with an asthenospheric origin for the parental magmas and so, consistent with mass balance calculations, it is inferred that they were derived from the lithospheric mantle.

A Geotransect from Dharimanna to Billimora across the Cambay and Narmada-Tapti Rift Basins, India

The rift basins of northwestern India are genetically related to the evolution of the west coast of India, which evolved in different stages due to the breakup of Africa, Madagascar, and the Seychelles. A geotransect covering these rift valleys provides significant information regarding their structures and evolution, essential to an understanding of the break-up history of the Indian plate. The Bouguer anomaly map of western India has delineated three sets of linear gravity anomalies representing the Kutch (ENE-WSW), Cambay (NNW-SSE), and Narmada-Tapti (ENE-WSW) basins connected perpendicularly from north to south. A crustal cross section along a geotransect from Dharimanna to Billimora (550 km) along the Cambay and across the Narmada-Tapti basins is presented, combining the known geology and tectonics with the seismic section and the modeling of gravity profiles. The resulting crustal model suggests a shallow Moho at a depth of 31 to 32 km, a high-density layer in the lower crust (3.1 g/cm3) at a depth of 25 to 26 km, and a low-density layer in the upper crust at a depth of 8 to 10 km. These are signatures of extensional regimes, suggesting them to be typical continental rift valleys. High heat flow and occurrences of seismic activities suggest that these rift valleys are still active to a limited extent. The entire Cambay basin is divided into several subbasins by transverse ridges that are fault controlled and may represent mafic, basic, and acidic intrusive bodies as found along its southwestern margin. The maximum thickness of Tertiary sediment varies from 2 to 3 km over the ridges to 5 to 6 km in the depressions. These sediments are underlain by Deccan Traps and Mesozoic (Cretaceous) sediments in certain parts. The Narmada-Tapti section south of the Cambay basin coincides with a gravity low that is a depression, and a major gravity high that extends for ~1000 km from the west coast of India to central India. It presently represents a median uplift in the form of the Satpura high caused by large-scale magmatism in the crust. It is suggested that although the breakup of Africa from India and related magmatism is primarily responsible for the evolution of the Kutch rift basin and the Gulf of Kutch, the latter two events—namely the breakup of Madagascar and the Seychelles-have controlled the evolution of the Cambay and the Narmada-Tapti rift basins from the Gulf of Cambay. These breakups of Indian plate have produced a considerable amount of magmatism in the crust in the form of intrusions and underplating in northwestern India, giving rise to several large-wavelength gravity and magnetic anomalies.

Rb-Sr ages of granitic rocks in southeastern China and their tectonic significance

Rb-Sr age data and initial Sr 87 /Sr 86 ratios (I values) are reported for the Mesozoic granitic batholiths along the coast of southeastern China. Two major thermal episodes are recognized: 90 to 120 m.y. B.P., derived from the mineral isochrons, and 165 ± 13 (2σ) m.y. B.P., derived from a whole-rock isochron. These episodes appear to be correlative with periods of rapid spreading of the Mesozoic Pacific Ocean floor and embrace two orogenic phases of the Yenshan orogeny. The I values of granitic gneisses (0.7060 to 0.7159) vary systematically with the Rb/Sr ratios of their whole-rock samples. The high I value (0.7112) of a pegmatite dike cutting a granitic gneiss in Chinmen suggests that the pegmatite is derived from remelting of upper crustal rocks. The narrow range of I values for more mafic intrusive bodies in Matsu (0.7065 to 0.70695) suggests that they are genetically related and are probably derived from an upper mantle source and have some crustal contamination. An un-metamorphosed granitic sample from Changlo has a mineral isochron age of about 120 m.y., which is interpreted as the time of intrusion. Its rather low I value (0.70546) suggests that the magma source is likely in the upper mantle. The Yenshan orogenic belt in southeastern China may belong to the inner belt of the Pacific-type orogeny; it is characterized by polycyclic deformation, magmatism, and regional metamorphism. Unlike a classical geosynclinal regime, the magmatic activities in this region were not immediately preceded by a major marine geosynclinal subsidence. The apparent absence of marine deposits after Triassic time suggests that marine transgression is not a universal result of rapid sea-floor spreading. The extensive development of magmatic belts along the eastern margin of the Asian continent appears to require large-scale consumption of oceanic lithosphere at the edge of the continental plate. The age distribution and structural trend data of the magmatic rocks suggest that the presumed subduction zone may have dipped westward or northwestward in the western Pacific region during late Mesozoic time.

Gravity Measurements in the Vicinity of Georges Bank

A total of 97 new bottom gravity measurements on the continental shelf in the vicinity of Georges Bank was reduced to the simple Bouguer anomaly, using a density of 2.80 gm per cm 3 for the correction. Results help substantiate the presence of mafic and felsic intrusive bodies along the northern edge of the bank. A gravity low near the center of the bank, trending northeast, corresponds to the Georges Bank trough. An abrupt change in the gravity gradient near the southeast edge of the bank probably represents the thinning of the crust from continental to oceanic thicknesses. Differences in gravity gradient support the suggestion of a fault along the northern edge of the bank.

AN AEROMAGNETIC AND AERORADIOACTIVITY SURVEY OF LIBERIA, WEST AFRICA

A 140,000 km aeromagnetic and total‐count gamma radiometric survey was made over Liberia in 1967–68 along north‐south lines spaced 0.8 km over land and 4 km over the continental shelf. The data approximately delineate the boundary between the Liberian (ca. 2700 m.y.) age province in the northwestern two‐thirds of the country, and the Pan‐African (ca. 550 m.y.) age province in the coastal area of the northwestern two‐thirds of the country, as well as a boundary marking the northwest extent of the isoclinally folded paragneisses and migmatites deformed within the Eburnean (ca. 2000 m.y.) age province in the southeast one‐third. A zone of diabase dikes about 90 km inland can be traced, parallel to the coast from Sierra Leone to Ivory Coast on the basis of the magnetic data. Another zone of diabase dikes about 185 m.y. old is located along the coastal area and beneath the continental shelf parallel to the coast northwest of Greenville. Intrusion of these dikes probably coincides with the separation of Africa from North and South America. The magnetic data suggest basins of sedimentary rocks possibly 5 km thick on the continental shelf. The map indicates high‐amplitude magnetic anomalies greater than 600 gammas; some reach amplitudes as great as 18,000 gammas over iron formation and about 1800 gammas over mafic and ultramafic intrusive bodies. The radioactivity data have a background level less than 100 counts per second (cps) over mafic granulite‐facies rocks and unmetamorphosed sedimentary rocks in the coastal area. Granitic rocks have the greatest variation. The central area of the country has the highest background radiation level with large areas above 250 cps; the level in the eastern one‐third of the country is low. These data are proving quite useful in reconnaissance geologic mapping. All anomalies over 500 cps are shown; some reach amplitudes over 750 cps. Total‐count radiation levels have a significant correlation with percent [Formula: see text] in bedrock analyses, but anomalous amounts of Th and U must be present to account for the highest amplitude anomalies. A few specific anomalies have been correlated with concentrations of monazite and zircon in bedrock as well as in beach deposits.

AN AEROMAGNETIC PROFILE FROM ANCHORAGE TO NOME, ALASKA

A total‐intensity profile was obtained on a 500‐mile flight by a U. S. Geological Survey airplane from Anchorage to Nome, Alaska, on May 4, 1954. The average flight altitude was 6,000 ft above sea level except over the Alaska Range where the flight altitude was 9,000 ft. This profile crossed eight of the major tectonic elements of Alaska at right angles to their trend and gives valuable regional information in an area where other geophysical and geological information is scarce or lacking. The profile has a net gradient downward to the northwest, most of which is ascribed to the component of the earth’s main magnetic field along the flight traverse. The great variety of magnetic anomalies which are superimposed on this gradient originate from variations in lithology along the traverse. All the magnetic anomalies, except a large one over the Yukon River, are caused by magnetic rocks at or near the surface. The magnetic profile may be divided into four major segments and nine subsegments, each having a characteristic magnetic pattern. Most of these can be related to a tectonic unit. The large plutons of the Talkeetna geanticline are clearly defined by a group of anomalies having the highest amplitudes of any on the profile. The Matanuska geosyncline to the east is represented by a 25‐mile section of sloping profile consistent with a thick sedimentary section but indicating that the geosyncline is comparatively narrow near Anchorage. The 200‐mile central magnetic segment is relatively free from all but very minor anomalies. This segment includes the Alaska Range geosyncline, the Tanana geanticline, and the Kuskokwim geosyncline; showing only slight magnetic contrasts between each of these elements. The two geosynclines either have thick Mesozoic sedimentary sections or have underlying crystalline rocks which are low in magnetic susceptibility at shallow depths. The rocks of the geanticline have a low but not negligible magnetic susceptibility and are predominantly Paleozoic sedimentary rocks. A single 300‐gamma anomaly on the west edge of the central segment is caused by a small, mafic intrusive body in the Paleozoic metamorphic rocks of Mt. Hurst. West of this anomaly the profile consists of a series of small sharp anomalies which are probably caused by Paleozoic metavolcanic rocks of the Ruby geanticline. The second largest anomaly on the profile is in the Koyukuk geosyncline over the Yukon River. The source is calculated to be more than a mile deep and may be an intrusive body at least 15 miles wide. This anomaly is flanked by 20‐mile sections of flat or sloping profile which indicate areas of thick sedimentary rocks, particularly in the region west of the Yukon River. The 150‐mile Norton Sound magnetic segment on the western end of the profile consists of many closely spaced anomalies produced by rocks which are either volcanic or similar to the Seward complex. Of the four Cenozoic basins or lowlands crossed by the profile, three are underlain by rocks of moderate to high magnetic susceptibility at shallow depths. These are the Cook Inlet basin, part of which overlaps rocks of the Talkeetna geanticline, the Innoko basin of central Alaska which overlies the rocks of the Ruby geanticline, and the Norton basin, in which sedimentary deposits are thin or absent. The fourth, the Minchumina basin, is underlain by the low‐susceptibility rocks at the Tanana geanticline, which are also probably close to the surface.

GEOLOGIC EVOLUTION OF THE BEARTOOTH MOUNTAINS, MONTANA AND WYOMING

Research Article| September 01, 1959 GEOLOGIC EVOLUTION OF THE BEARTOOTH MOUNTAINS, MONTANA AND WYOMING: PART 3. GARDNER LAKE AREA, WYOMING RAE L HARRIS, JR. RAE L HARRIS, JR. DEPARTMENT OF GEOLOGY, TEXAS TECHNOLOGICAL COLLEGE, LUBBOCK, TEXAS Search for other works by this author on: GSW Google Scholar GSA Bulletin (1959) 70 (9): 1185–1216. https://doi.org/10.1130/0016-7606(1959)70[1185:GEOTBM]2.0.CO;2 Article history received: 27 May 1958 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation RAE L HARRIS; GEOLOGIC EVOLUTION OF THE BEARTOOTH MOUNTAINS, MONTANA AND WYOMING: PART 3. GARDNER LAKE AREA, WYOMING. GSA Bulletin 1959;; 70 (9): 1185–1216. doi: https://doi.org/10.1130/0016-7606(1959)70[1185:GEOTBM]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Gardner Lake area is one of the key areas being studied in detail in the Beartooth Mountains, Montana and Wyoming. Foliation, compositional banding, and rock units indicate open southward-plunging folds traceable through metasediments and migmatites into granitic gneisses. The attitude of axial-plane foliation in highly contorted metasediments is found also in foliation trends of the gneisses. Complete gradations from metasediments to pink leucocratic gneiss are indicated by distribution of mappable units, abundance and composition of feldspars and ferromagnesian minerals, percentage and character of rounded and overgrown zircons, length-width ratios of zircons, and variations in chemical analyses. Replacement in situ by hot alkaline solutions seems the most likely origin for the granitic gneisses. Boudinage and rotation of amphibolite blocks within highly contorted feldspathized strata denote regional metamorphism prior to granitization. Mafic intrusive bodies are pregranitization, penegrantization, and postgranitization in age, and their major trends, northwest and northeast, suggest that the present topographic expression is controlled by structural elements developed during feldspathization. Seven K-A and Rb-Sr ages on widely dispersed micas and feldspars indicate this event as 2.7 billion years ago. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.