Cenozoic Igneous Rocks Research Articles

The palaeomagnetism of Lesbos, NE Aegean, and the eastern Mediterranean inclination anomaly

Summary Palaeomagnetic results for 44 sites in 16–22 Ma volcanic rocks from Lesbos, NE Aegean, yield a mean pole at 81.8°N, 178.1°E, K = 9.0, A95 = 7.6°. The mean direction for these sites (D = 4.3°, I = 48.5°, k = 10.8, α95 = 6.9°) is 5.9° ± 6.1° shallower than the reference direction for Miocene Lesbos calculated from Besse & Courtillot (1991). Combining these new data with previous work yields a mean inclination that is 5.6° ± 4.7° too shallow. Experimental problems, magnetic anisotropy, the magnetic terrain effect, geomagnetic anomalies, and problems with the reference path all seem unable to account for this observation. Shallow inclinations are a common feature of the Aegean region: 17 of 18 palaeomagnetic studies on Cenozoic igneous rocks have returned an inclination that is shallower than expected. Northward motion of the Aegean block by ∼500 km with respect to northern Europe would account for this observation.

The palaeomagnetism of Lesbos, NE Aegean, and the eastern Mediterranean inclination anomaly

Palaeomagnetic results for 44 sites in 16–22 Ma volcanic rocks from Lesbos, NE Aegean, yield a mean pole at 81.8°N, 178.1°E, K = 9.0, A95 = 7.6°. The mean direction for these sites (D = 4.3°, I = 48.5°, k = 10.8, α95 = 6.9°) is 5.9° ± 6.1° shallower than the reference direction for Miocene Lesbos calculated from Besse & Courtillot (1991). Combining these new data with previous work yields a mean inclination that is 5.6° ± 4.7° too shallow. Experimental problems, magnetic anisotropy, the magnetic terrain effect, geomagnetic anomalies, and problems with the reference path all seem unable to account for this observation. Shallow inclinations are a common feature of the Aegean region: 17 of 18 palaeomagnetic studies on Cenozoic igneous rocks have returned an inclination that is shallower than expected. Northward motion of the Aegean block by ∼500 km with respect to northern Europe would account for this observation.

Cretaceous and Tertiary sedimentary, magmatic, and tectonic evolution of north-central Sonora (Arizpe and Bacanuchi Quadrangles), northwest Mexico

The Arizpe and Bacanuchi Quadrangles provide a geologic history representative of the north-central part of Sonora, where lithologies are dominated by late Mesozoic and Cenozoic igneous rocks. In this study, new geologic mapping, 40Ar/39Ar dating, and geochemical analyses have been combined to provide a stratigraphic framework for this area. Ten lithostratigraphic units and several igneous and tectonic events can be recognized. The oldest outcropping rocks are Lower Cretaceous strata of the Bisbee Group, which along with the Picacho conglomerate record a middle Cretaceous compressive tectonic event and associated sedimentation. Laramide igneous activity is widespread and represented by (1) highly altered andesitic flows and volcaniclastic rocks (Arroyo Alcaparros andesitic rocks) of late Campanian to Maastrichtian age, (2) less altered andesitic and dacitic flows (Cerro Las Jarillas volcanic rocks) of late Paleocene age, and the intrusive bodies of (3) Sierra El Manzanal granodiorite and (4) Rancho Vaqueria quartz monzonite. The Sierra El Manzanal granodiorite was emplaced at ca. 68 Ma on the basis of a 40Ar/39Ar biotite age (67.97 ± 0.19 Ma) and cooled relatively rapidly according to less precise 40Ar/39Ar hornblende and K-feldspar ages from the same sample (64.8 ± 1.0 Ma and 62.8 ± 0.3 Ma, respectively). The Cerro Las Jarillas volcanic rocks are slightly younger (40Ar/39Ar biotite age of 58.67 ± 0.17 Ma). The Rancho Vaqueria quartz monzonite was emplaced at ca. 57 Ma (40Ar/39Ar biotite age of 56.73 ± 0.14 Ma and a less precise 40Ar/39Ar hornblende age of 55.0 ± 0.7 Ma); a protracted cooling history of this pluton is indicated by the age spectrum of K-feldspar from the same sample. A probable magmatic lull and denudation seem to have occurred between middle and late Eocene time and probably until the early Oligocene. Subsequently, rhyolitic to mafic volcanism began close to late Oligocene time and lasted until the early Miocene. Felsic volcanism is represented by the Cerro Cebadehuachi volcanic rocks, from which 40Ar/39Ar hornblende ages of 27.25 ± 0.09 and 27.32 ± 0.06 Ma and a biotite age of 26.97 ± 0.06 Ma were obtained at three different localities. The Mesa Pedregosa volcanic rocks represent the transition to younger, mafic volcanic activity that occurred during the late Oligocene, as indicated by a sanidine 40Ar/39Ar age of 25.48 ± 0.05 Ma. This late Oligocene and early Miocene magmatism was paired by two episodes of extensional deformation. The first phase is characterized by northwest-striking normal faults and folds, which expose the deepest structural levels of the area, and by the related basin fill, the Bacanuchi conglomerate. The second phase is represented by north-striking normal faults and by the syntectonic basin fill, the Arizpe conglomerate. Basaltic andesite volcanic flows at the base of the Arizpe conglomerate yielded 40Ar/39Ar (whole-rock) ages of 23.52 ± 0.17 and 21 ± 0.20 Ma. The extensional deformation (27 to 23 Ma) in the study area is coeval with the development of metamorphic core complexes in neighboring areas of Sonora and with the onset of extension in southern Sonora. The mafic volcanic rocks and clastic sedimentary units associated with this extension resemble the basin fills that in other parts of Sonora are assigned to the Baucarit Formation. Geochemical information from samples representing each of the igneous events displayed high-K calc-alkalic and mostly metaluminous compositions. The older units including the Arroyo Alcaparros andesitic rocks, the Cerro Las Jarillas volcanic rocks, the Sierra El Manzanal granodiorite, and the Rancho Vaqueria quartz monzonite are characterized by steep chondrite-normalized REE (rare earth element) slopes and generally well-developed negative Eu anomalies, suggesting garnet and plagioclase removal in the source. The younger igneous events including the Cerro Cebadehuachi and Mesa Pedregosa volcanic rocks, and the basaltic flows associated with the Arizpe conglomerate, showed basin-shaped REE slopes with no Eu anomalies, suggesting clinopyroxene or amphibole fractionation.

Lower crustal and upper mantle xenoliths along the Cheyenne belt and vicinity

Lower-crustal and upper-mantle xenoliths entrained in Phanerozoic igneous rocks in the northern Southern Rocky Mountains provide the only direct means of studying the petrologic and geochemical nature of the deep-continental lithosphere in this region. Petrographic and geochemical features of peridotite xenoliths from the State Line kimberlite district, near the Cheyenne belt, suggest that during Devonian kimberlite emplacement, infertile, cryptically metasomatized, Archean mantle dipped south beneath a distinctly more fertile, and veined, lithospheric mantle associated with the Proterozoic continental crust of the Colorado province. Lower-crustal xenoliths recovered from the State Line kimberlites and late Cenozoic ultrapotassic igneous rocks at Leucite Hills (north of the Cheyenne belt) are dominated by mafic granulites. This indicates that both the Archean and Proterozoic lower-continental crust was, and may remain today, mafic in composition. Existing major-element data support a tholeiitic basalt precursor for the mafic granulites associated with the Colorado province. Unfortunately, despite its importance for constraining models of lithospheric structure in the northern Rocky Mountains, detailed information regarding the age, trace element and isotopic compositions, or physical attributes of the mafic xenoliths is presently not available. In order to evaluate the age and composition of continental lithosphere in this region and to assess the possible role of mafic lower-crustal rock types as sources for Laramide magmatism we outline a plan (as part of the Rocky Mountain Continental Dynamics project) to obtain geochemical and geochronological information from xenoliths recovered at the State Line kimberlite district and Leucite Hills.

Nd and Sr isotopic constraints on the origin of igneous rocks resulting from the opening of the Japan Sea, southwestern Japan

Strontium and Nd isotopic compositions and trace element abundances were determined for Cretaceous to late Cenozoic igneous rocks from the Japan Sea side of Southwest Japan in order to investigate the effect of the opening of the Japan Sea on igneous activity. The 87Sr/86Sr ratios for both high and low silica rocks decrease with decreasing age since the middle Miocene, when the opening occurred. Similarly, 143Nd/144Nd values for these rocks increase with decreasing age, and are negatively correlated with 87Sr/86Sr ratios. A two-component mixing process can best account for these isotopic and chemical characteristics. One end-member is likely the subcontinental lithospheric mantle (SCLM) and its derivative mafic to intermediate materials which had ɛNd values of around +3. The other endmember consists of mafic to intermediate rocks with low ɛNd values (e.g., −8), probably located in the lower crust. The mantle upwelling associated with the opening of the Japan Sea did not supply typical MORB or MORB-source materials to the crust, but did provide the heat that caused the melting of lithospheric mantle and lower crust.

Lead isotopic compositions of Neogene volcanic rocks from the Aegean extensional area

Lead isotope composition has been determined for a diverse suite of Cenozoic igneous rocks in the central and north Aegean Sea. These include voluminous Miocene shoshonitic and calc-alkali volcanic rocks and correlative plutonic phases; middle Miocene adakitic (high Mg) andesite and associated rhyolite; and middle Miocene to Quaternary ne-normative basalt, trachyte and K-rich andesite. The trace-element contents of most of the studied rocks suggests that Pb-isotopic composition is not a result of upper-crustal contamination, but rather is a feature of the mantle source for K-rich rocks. This mantle source is represented by primitive mafic to intermediate volcanic rocks with 87Sr86Sr ≈ 0.7095, high 207Pb204Pb and 208Pb204Pb, >1000 ppm Sr and Ba, and >35 ppm Pb. The high 207Pb204Pb requires a metasomatic origin that lowered U/Pb and increased Th/Pb, followed by a second-stage residence time of >1 Ga. This K-rich magma was generated as a result of late Tertiary extension of the Aegean. In the South Aegean Arc, there is more evidence for crustal contamination, either from present crust or from subduction of Nile-derived sediment.

Regional study of mineral resources in Nevada: Insights from three-dimensional analysis of gravity and magnetic anomalies

A three-dimensional interpretation of the basins of Nevada was developed based on statewide data bases of digital-gravity, magnetic, geologic, well, and topographic information. An iterative technique was applied to isostatic residual gravity anomalies in Nevada in order to define the location and shape of pre-Tertiary basement and to produce a gravity map that reflects variations of density within the pre-Tertiary basement. The dominant feature of the basement gravity of Nevada is an enormous area of low gravity that spans the entire state between latitudes 37°N and 40.5°N. This regional low strongly correlates with the distribution of middle and late Tertiary volcanic rocks and may reflect silicic intrusions within the mid-crust and upper crust that are the counterparts of volcanic rocks at the surface. Although 80% of Nevada is covered by Cenozoic deposits, these deposits are thicker than 1 km over only about 20% of the state. The remaining 60% of Nevada may have pre-Tertiary basement rocks within reach of current mineral-exploration techniques. Aeromagnetic profiles from the National Uranium Resource Evaluation (NURE) were analyzed in order to produce a map showing the location of shallow magnetic sources in Nevada. This analysis shows that 46% of the state has magnetic sources, generally Mesozoic and Cenozoic igneous rocks, within 1 km of the surface. A linear magnetic anomaly in north-central Nevada has been interpreted by others as a rift zone active during middle Miocene time. The rift also is evident in NURE magnetic profiles, but our interpretation suggests that the magnetic expression of the rift continues south-southeast with similar strike to at least 38°N and perhaps to the amagmatic zone (lat. 37°N). The survival since the middle Miocene of this narrow crustal feature, essentially linear over a distance of 500 km, is difficult to interpret in light of later Basin and Range deformation. Our analysis of gravity anomalies shows that many deep Cenozoic basins are located near the rift, yet only two basins cut across it, and at least five others change strike near the rift, as if to avoid it. The rift may have remained linear because it is associated with crustal structures that acted to resist subsequent deformation.

Potassium/argon ages of Cenozoic igneous rocks from eastern Shimane Prefecture—Oki Dozen Island, Southwest Japan and the Japan Sea opening

New K/Ar dates for igneous rocks of the Shimane-Oki district of SW Japan span the Middle Miocene to Late Pleistocene interval. They largely confirm the stratigraphy of this region as indicated by Kobayashi et al. [1982. Geological Map of Shimane Prefecture (1:200,000). Department of Geology, Shimane University (in Japanese with English abstract)]. The three dated Middle Miocene units (Omori and Ushikiri Formations and Dolerites) represent tholeiitic activity coincident with and perhaps causally related to the opening of the Japan Sea. Ages for the Ushikiri and Matsue Formations indicate little overlap of igneous events in contrast to conclusion of other workers. Matsue Formation basalts are compositionally diverse yet have identical eruptive ages. This, and the eruption of calc-alkalic and alkalic rocks throughout t the Pliocene-Pleistocene argue for derivation of magma from a variety of sources Subduction influence was probably provided by the Pacific Plate until the Upper Miocene, then the Philippines Sea Plate, although this aspect requires further work. Alkalic volcanism on Oki Dozen Island largely preceded eruption of alkalic volcanics on neighbouring Oki Dogo Island.

Migration of Tertiary volcanism in the southwestern United States and subduction of the Mendocino fracture zone

A compilation of 417 isotopic dates on mid and late Cenozoic igneous rocks from the southwestern United States shows that volcanism migrated northward with time. The principal locus of volcanism at any given time was an east-west band that corresponded closely with the calculated position of the subducted Mendocino fracture zone (MFZ) under the North American plate. This correspondence supports the theory that volcanism was triggered by subduction of the MFZ, which was a major (1 km) north-facing topographic step in the Farallon plate. Both volcanism and the MFZ moved northward at about 3.1 cm/year. Andesites and rhyolites show close correspondence to MFZ passage, but many basalts were erupted significantly later. Cooling dates on basement rocks in southern Arizona cluster at the time the MFZ passed under that area. Earlier models proposing rapid late Tertiary steepening of the subducted Farallon plate were based on a westward sweep of volcanism. Our compilation shows no evidence for such a westward sweep.

Palaeomagnetism and K [sbnd]Ar age of Mesozoic and Cenozoic igneous rocks from Antarctica

A new analysis of palaeomagnetic data for igneous rocks from Deception Island, 25 de Mayo Island (King George Island) and Cape Spring, are given. K-Ar age determinations indicate that most of the igneous samples from 25 de Mayo Island included in the palaeomagnetic study are of Late Mesozoic/Early Tertiary age. The significance of these palaeomagnetic-radiometric data on the hypothesis of oroclinal bending of the Antarctic Peninsula and on the apparent polar movement of Antarctica is discussed. The positions of palaeomagnetic poles for the Andean igneous complex indicate that there has not been any apparent post-Late Cretaceous/Early Tertiary oroclinal bending in the Antarctic Peninsula from 74°S to 62°S. A comparison of the positions of palaeomagnetic poles for Antarctica and Australia suggests that the direction of apparent polar movement relative to Antarctica reversed after the Miocene.

Genesis of ore deposits in the San Juan volcanic field, southwestern Colorado; lead isotope evidence

In the San Juan volcanic area of southwestern Colorado, the isotopic composition of lead in ores and ore prospects of Cenozoic age ranges widely: 17.72 to 21.13 for 206 Pb/ 204 Pb; 15.50 to 15.81 for 207 Pb/ 204 Pb; and 37.21 to 38 for 208 Pb/ 204 Pb. Examination of the lead isotope data indicates that once deposition of lead minerals begins, further exchange of lead between fluid and wall rock is insignificant. This conclusion is supported by the relatively constant isotopic composition of the lead in these ores, which is not affected by the grade of ore mineralization or type of wall rock. The values of 206 Pb/ 204 Pb in some vein-type deposits exceed the maximum value known for all Mesozoic and Cenozoic igneous rocks of the Rocky Mountain region. These isotopic relations show that if ore-forming solutions are composed of meteoric water, as indicated in studies of light stable isotopes, they must have penetrated deep enough to acquire lead from Precambrian rocks or sediments derived from them. The fact that some of these vein ores are now in Cenozoic igneous rocks indicates the ore fluid had an upward vertical component to its movement. The lead isotope data therefore support a circulating cell hypothesis for these kinds of ores, as suggested by many recent studies of light stable isotopes as related to mineralization.Some other deposits (Summitville, Jasper, Red Mountain district) have values of 206 Pb/ 204 Pb similar to those of the large volumes of altered rock that enclose them ( 206 Pb/ 204 Pb approximately 18.5), suggesting that in places the lead may have been locally derived by leaching of the adjacent rocks or from magmatogenic fluids.When the lead isotope data are treated in detail, the rocks and galenas of the Platoro caldera complex, of the central San Juan caldera complex, and of the Baughman Creek center appear to contain significant components derived from 1,400- to 1,500-m.y.-old and 1,700- to 1,800 m.y.-old source materials. These also are the ages of the two main groups of rocks that comprise the Precambrian basement under the San Juan volcanic area. Although the data from the western San Juan caldera complex scatter considerably, the only obvious source for the lead seems to be the 1,700- to 1,800-m.y.-old rocks or detritus of such age in Phanerozoic sediments. Where the involvement of 1,400- to 1,500-m.y.-old sources is greatest, the Th/U ratio in the source material is small (calculated to be about 0.7), whereas the Th/U ratio of the 1,700- to 1,800-m.y.-old source material appears to be 2.3 to 3.3. The largest Th/U values are from the western San Juan caldera complex.

East-west patterns of Cenozoic igneous rocks, aeromagnetic anomalies, and mineral deposits, Nevada and Utah

In Nevada and western Utah, Cenozoic igneous rocks within several age increments crop out in arcuate, generally east-trending belts, each successively younger to the south. Broad aeromagnetic highs with superimposed short-wavelength anomalies are associated with some of these outcrop belts. Mineral deposits are aligned along the belts in easternmost Nevada and western Utah. The east-west patterns are the result of a southward-migrating front of igneous activity that, in Nevada and Utah, started about 43 to 34 m.y. ago near lat 40°N and ended about 17 to 6 m.y. ago near lat 37°N. During any one time interval, igneous activity was concentrated near the leading edge of the east-trending front. The volcanic front may be related to igneous activity localized along a southward-propagating transverse break or structural warp in a subducting plate.

Episodic Volcanism in the Central Oregon Cascade Range

Radiometric dating of late Cenozoic igneous rocks of the central Oregon Cascade Range has shown that volcanic activity has been strongly episodic, with most lavas erupting during short intervals of 1 or 2 m.y. The strongest activity occurred during middle Miocene time concurrently with eruption of the Columbia River Basalt. Subsequent episodes followed at about 5-m.y. intervals. Preliminary data from other parts of the circum-Pacific area suggest that these same episodes occurred in unison over a large part of the Earth.

K/Na ratio of Cenozoic igneous rocks of the western United States

The potassium and sodium content of chemically analysed Cenozoic igneous rocks from about 150 areas of the western United States has been examined. For each area a plot of the molecular proportion K 2O (K 2O + Na 2O) [Niggli's k-value] is shown, and the projected k-value determined at 50 and 60 weight per cent SiO 2. The k-values are plotted and contoured on maps of the western United States. These maps show that potassium is least abundant relative to total alkali (when rocks of the same SiO 2 content are compared) in a zone along the Pacific Coast, becomes more abundant eastward, and is highest in the Colorado Plateau and Northern Rocky Mountains. These k-value variations can be related to regional variations in the abundance of certain trace elements and of different types of older granitic rocks, and to Bouguer gravity maps. This correspondence indicates that the alkali ratio of Cenozoic igneous rocks is closely related to the character of the crust where the rocks are formed.