Late Cretaceous Volcanic Rocks Research Articles

Late cretaceous mantle plume activity in Ceno-Tethys: evidences from the Hamrani volcanic rocks, western Pakistan

Basaltic pillow lavas from near the Muslim Bagh town in northern Balochistan are found in the tectonic slivers of the Bagh Complex, northwest of Muslim Bagh Ophiolite. These volcanic rocks are mainly basanites and tephrites. Their petrography and chemistry suggest that these belong to the mildly–strongly alkaline, intra-plate volcanic rock series. Their low Mg # and low Cr, Ni, and Co contents suggest that the parent magma of these volcanic rocks was not directly derived from a partially melted mantle source but have been modified by fractionation en route to eruption. Their primordial mantle-normalized trace element patterns show enrichment in large ion lithophile elements (LILE) and in high field strength elements (HFSE), with marked positive Nb anomalies, and further confirm their within-plate geochemical signatures consistent with an enriched mantle source. Modeling of their Zr versus Zr/Y trends suggest that these volcanic rocks were derived from about 10–15 % melting of an enriched OIB-style mantle source. It is suggested that these Late Cretaceous intra-plate volcanic rocks may represent the mantle plume activity possibly of the Reunion hotspot. These may have erupted on the Ceno-Tethys Ocean floor prior to the passage of Indian plate over it with subsequent tectonic imbrication during Ceno-Tethyan closure and collision with the Eurasian margin.

Multi-stage volcanic activities and geodynamic evolution of the Lhasa terrane during the Cretaceous: Insights from the Xigaze forearc basin

The history of volcanic activity of the Gangdese arc in southern Tibet during the Cretaceous remains poorly known due to the intense erosion of the arc. Here we present zircon U–Pb ages, trace element and Hf isotopic data of tuffs and volcanic conglomerates from the Chongdoi and the Ngamring Formation in the Xigaze forearc basin. Three tuff samples from the Chongdoi Formation yield zircon U–Pb ages around 112Ma. Such ages could be taken as their depositional ages, indicating that detrital clasts of the Chongdoi Formation were deposited at that time. One andesitic conglomerate sample from the Ngamring Formation was dated at ca. 105Ma, the other rhyolitic conglomerate sample and one tuff sample were dated to be ca. 95Ma and 91Ma, respectively, suggesting that this formation was formed during the late Albian–late Turonian. All zircons illustrate I-type granitoid characteristics and possess low Ti-in-zircon temperatures (<800°C). Zircon εHf(t) values of tuffs from the Chongdoi Formation define two groups: (1) the first group of 117–110Ma displays large positive εHf(t) values (+12.1 to +17.1), larger than those of the tuff from the Ngamring Formation (+6.0 to +10.2); (2) the second group of 119–111Ma yields negative to small positive εHf(t) values (−4.5 to +1.1). All above observations indicate that their host rocks were derived from the juvenile materials with significant input of fluids. Combined with extant data, the first group and the tuff, volcanic conglomerates from the Ngamring Formation are mostly likely derived from the eroded Cretaceous volcanic rocks in the Gangdese arc, while the second group is likely sourced from the late Early Cretaceous volcanic rocks in the central Lhasa terrane. Our data confirm the presence of Cretaceous volcanism in the Gangdese arc, suggesting that the Neo-Tethyan Ocean lithosphere was most likely subducted northerly in a normal angle rather than in a manner of low-angle subduction.

Spatial–temporal relationships of Mesozoic volcanic rocks in NE China: Constraints on tectonic overprinting and transformations between multiple tectonic regimes

LA-ICP-MS zircon U–Pb ages and geochemical data are presented for the Mesozoic volcanic rocks in northeast China, with the aim of determining the tectonic settings of the volcanism and constraining the timing of the overprinting and transformations between the Paleo-Asian Ocean, Mongol–Okhotsk, and circum-Pacific tectonic regimes. The new ages, together with other available age data from the literature, indicate that Mesozoic volcanism in NE China can be subdivided into six episodes: Late Triassic (228–201Ma), Early–Middle Jurassic (190–173Ma), Middle–Late Jurassic (166–155Ma), early Early Cretaceous (145–138Ma), late Early Cretaceous (133–106Ma), and Late Cretaceous (97–88Ma). The Late Triassic volcanic rocks occur in the Lesser Xing’an–Zhangguangcai Ranges, where the volcanic rocks are bimodal, and in the eastern Heilongjiang–Jilin provinces where the volcanics are A-type rhyolites, implying that they formed in an extensional environment after the final closure of the Paleo-Asian Ocean. The Early–Middle Jurassic (190–173Ma) volcanic rocks, both in the Erguna Massif and the eastern Heilongjiang–Jilin provinces, belong chemically to the calc-alkaline series, implying an active continental margin setting. The volcanics in the Erguna Massif are related to the subduction of the Mongol–Okhotsk oceanic plate beneath the Massif, and those in the eastern Jilin–Heilongjiang provinces are related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent. The coeval bimodal volcanic rocks in the Lesser Xing’an–Zhangguangcai Ranges were probably formed under an extensional environment similar to a backarc setting of double-direction subduction. Volcanic rocks of Middle–Late Jurassic (155–166Ma) and early Early Cretaceous (145–138Ma) age only occur in the Great Xing’an Range and the northern Hebei and western Liaoning provinces (limited to the west of the Songliao Basin), and they belong chemically to high-K calc-alkaline series and A-type rhyolites, respectively. Combined with the regional unconformity and thrust structures in the northern Hebei and western Liaoning provinces, we conclude that these volcanics formed during a collapse or delamination of a thickened continental crust related to the evolution of the Mongol–Okhotsk suture belt. The late Early Cretaceous volcanic rocks, widely distributed in NE China, belong chemically to a low- to medium-K calc-alkaline series in the eastern Heilongjiang–Jilin provinces (i.e., the Eurasian continental margin), and to a bimodal volcanic rock association within both the Songliao Basin and the Great Xing’an Range. The volcanics in the eastern Heilongjiang–Jilin provinces formed in an active continental margin setting related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent, and the bimodal volcanics formed under an extensional environment related either to a backarc setting or to delamination of a thickened crust, or both. Late Cretaceous volcanics, limited to the eastern Heilongjiang–Jilin provinces and the eastern North China Craton (NCC), consist of calc-alkaline rocks in the eastern Heilongjiang–Jilin provinces and alkaline basalts in the eastern NCC, suggesting that the former originated during subduction of the Paleo-Pacific Plate beneath the Eurasian continent, whereas the latter formed in an extensional environment similar to a backarc setting. Taking all this into account, we conclude that (1) the transformation from the Paleo-Asian Ocean regime to the circum-Pacific tectonic regime happened during the Late Triassic to Early Jurassic; (2) the effect of the Mongol–Okhotsk suture belt on NE China was mainly in the Early Jurassic, Middle–Late Jurassic, and early Early Cretaceous; and (3) the late Early Cretaceous and Late Cretaceous volcanics can be attributed to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent.

Isotope geochemistry (O, H and Sr) of Late Cretaceous volcanic rocks, Haţeg Basin, South Carpathians, Romania

AbstractOxygen and strontium isotopic ratios belonging to Late Cretaceous volcaniclastic deposits of the Haţeg Basin, South Carpathians, are documented here for the first time. The analyses performed on mineral concentrates suggest that associated magmas account for an assimilation–fractional crystallization trend with 87Sr/86Sr ratios of between 0.705 and 0.706, and a large range of δ18O up to 16‰, and a trend with higher 87Sr/86Sr ratios (0.707–0.708) but lower δ18O (8.2–7.9‰) values for leucrocratic minerals, such as plagioclase and sanidine. The Sr–O modelling of the main trend, using mafic minerals (pyroxene and amphibole), indicate 1–3% source contamination associated with up to 20% crustal assimilation. Hydrogen isotopic composition of amphiboles, biotite and groundmass do not confirm any significant involvement of an external fluid, either hydrothermal or diagenetic.

Paleomagnetism of the Late Cretaceous volcanic rocks of the Shimaoshan Group in Yongtai County, Fujian Province

It has been debated whether there was southward movement of the South China Block (SCB) during the Cretaceous. In this study, a paleomagnetic investigation was carried out on the Late Cretaceous volcanic rocks (~88 Ma) of the Shimaoshan Group in Yongtai County, Fujian Province. Rock magnetic experiments showed that magnetite in pseudo-single-domain and multi-domain grain and hematite were predominant magnetic phases. Stepwise thermal demagnetization successfully isolated characteristic directional components at high-temperature interval (> 500°C) from 383 specimens in 19 sites, which yielded a paleomagnetic pole for the studied section at 83.1°N, 152.6°E (N = 19, A95 = 3.9°), and the scatter SB = 9.0. The Fisher distribution of virtual geomagnetic poles (VGPs) and the consistence of SB with the expected value at the 95% confidence level indicate that the yielded paleomagnetic pole is free of paleomagnetic secular variation influence. The new pole, which is well consistent with that from the Eurasian apparent polar wander path (APWP) curve, suggests no obvious southward movement of the sampling site during the Cretaceous. South China Block, Late Cretaceous, volcanic rock, paleomagnetic secular variation, paleomagnetic pole

Late Cretaceous volcanic rocks and associated granites in Gyeongsang Basin, SE Korea: Their chronological ages and tectonic implications for cratonic destruction of the North China Craton

The Gyeongsang Basin is the largest exclusively non-marine sedimentary basin in Korean Peninsula, located in southeastern part of the Peninsula. Its sedimentary setting, magma source, tectonic nature, and formation age are important to understand the Mesozoic tectonic evolution of not only the Sino-Korea Craton, but also the entire Northeast Asia. The Gyeongsang Supergroup, the basin fill, is divided into the Sindong, Hayang, and Yucheon Groups in ascending order. The Sindong and Hayang Groups are composed of sandstone, shale, minor amounts of conglomerate, and marl deposited in a non-marine environment, whereas the Yucheon Group is characterized by the dominance of volcanic rocks. Precise isotopic ages and geochemical analyses are scarce for the volcanic elements of this basin, restricting our understanding of its geodynamic setting and evolution. In this study, we present new U–Pb zircon ages from the volcanic rocks of the Yucheon Group and associated granites in the Gyeongsang Basin. Thirteen volcanic samples yielded ages ranging from 94.4 to 78.4Ma, and four granite samples give ages ∼72Ma. These data confirm that a major magmatic event occurred during Late Cretaceous, and the Gyeongsang volcano-sedimentary complex represents a rock association representing a Late Cretaceous island arc. In comparison with the Sambagawa metamorphic belt, the Jurassic Chichibu accretionary complex (AC) with Cretaceous strike-slip basin sediments, and the Cretaceous Shimanto mélange zone in southwest Japan, it is likely that above-motioned complexes constitute the trace of a continuous subduction-related island arc system in SE Korea extending from SW Japan. Although the peak ages of the magmatic rocks in the eastern part of the North China Craton (NCC) are 130±10Ma, indicating an important time of continental lithospheric thinning under the NCC, the ages of the Gyeongsang magmatic rocks are markedly younger. Late Cretaceous magmatism was very weak in the eastern NCC. Therefore, it is probable that Early–Middle Mesozoic lithospheric thinning beneath the eastern NCC is triggered by multiple tectonic factors, not only by the subduction of the Pacific plate suggested by some previous studies. In addition, 20 inherited/xenocrystic zircons from these volcanic rocks give Archean ages ranging from 2.5 to 2.6Ga, suggesting the existence of late Archean rocks in the Yeongnam Massif. The Yeongnam Massif is predominated by the latest Archean crustal growth at ∼2.50Ga, consistent with the characteristics of the North China Craton.

Petrochemistry of the Late Cretaceous-Paleogene igneous rocks in the Ikuno-Akenobe mines area, Southwest Japan

The Ikuno and Akenobe vein deposits are known to be typical subvolcanic type being hosted in the Late Cretaceous volcanic rocks in the Ikuno mine area and in Paleozoic-Mesozoic igneous and metamorphic rocks in the Akenobe mine area. The ore veins have mostly N-S, partly NW strikes and steep dips in the Ikuno mine, and are rich in silver and base metals historically. Tin was found in later years. The ore veins as a whole show a concentric zoning of the central Sn-Cu subzone, surrounded by Sn-Cu-Zn subzone, Zn subzone, Pb-Zn subzone, Au-Ag subzone and barren subzone. The Akenobe veins have generally NW strike swinging to NE in the western part and steep dips. The ores are polymetallic but shown by two different stages of the early Pb-Zn-Cu substage and the later W-Sn-Cu substage. Late Cretaceous volcanic formation of the Ikuno Group is composed of andesitic, dacitic and rhyolitic lavas and pyroclastic rocks. By the measurement of the magnetic susceptibility and microscopic observations, these volcanic rocks are found to belong generally to the I-type magnetite-series. These rocks are plotted K-rich calc-alkaline rocks in the variation diagram. Rich ores tend to occur associated with basaltic and rhyolitic dikes in the Ikuno mine. Cu- and Sn-rich ore fluids may be originated in the basaltic and rhyolitic magmas, respectively. Abundant dikes, mostly post-ore though, in the Akenobe mine indicate that the I-type magnetite-series magmatism with wide range of compositions prevailed in this mine area and supplied the ore fluids to the early stage base metal mineralizations. But the second stage of Sn-rich ore fluids may be derived from an I-type ilmenite-series granitic body still hidden.

New paleomagnetic results from the Lhasa block: Revised estimation of latitudinal shortening across Tibet and implications for dating the India–Asia collision

Knowing how and when the Tibetan Plateau formed is of global concern as high topography modifies climate circulation patterns and alters species migration paths. Previous paleomagnetic work suggested that intracontinental shortening from deformation related to the India–Asia collision played a major role in building the Tibetan Plateau. However, these studies were derived from red bed facies, which are often afflicted by inclination shallowing, leading to overestimates of continental shortening. We report new paleomagnetic data from the Lhasa block, the southern continental margin of Asia before the India–Asia collision. A high temperature, characteristic remanent magnetization (ChRM) was isolated from 43 sites of Late Cretaceous red beds, 32 sites of Late Cretaceous lava flows, and nine sites of Eocene tuffs. All the ChRM directions are significantly different from the present-day geomagnetic field, and the mean directions of red beds, lava flows and tuffs pass the fold test. The distribution patterns of ChRM directions derived from volcanic rocks are consistent with a secular variation model, suggesting that secular variation was likely averaged out. In contrast, the red bed ChRM directions exhibit an east–west elongated distribution pattern, indicative of inclination flattening. The mean ChRM inclination of the red beds, 23.5°±2.5°, is consistent with previous results, but significantly shallower than that of 41.9°±4.4° carried by the coeval lava flows. An elongation―inclination analysis of the red bed ChRM directions yielded an unflattened mean inclination of 39.7°, with 95% confidence limits between 37.7° and 41.9°. The paleolatitudes of the Late Cretaceous volcanic rocks and inclination-corrected sediments suggest about 10° less shortening occurred between Lhasa and Siberia than previous estimates, with total shortening lying between 3.3°±2.6° and 7.3°±5.4° since the Late Cretaceous. The mean ChRM inclination derived from the Eocene tuffs, 51.8°±5.2°, is indistinguishable from the expected reference direction. Our new results imply that India–Asia convergence was accommodated mainly through consumption of the India plate and yield a much younger age (ca. 43Ma) for the initial India–Asia collision than previously estimated by paleomagnetic data.

Origin and geochemical evolution of the Madeira‐Tore Rise (eastern North Atlantic)

The Madeira‐Tore Rise, located ∼700 km off the NW African coast, forms a prominent ridge in the east Atlantic. The age and origin of the rise are controversial. This study presents major and trace element, Sr, Nd, Pb, Hf isotope and 40Ar/39Ar age determinations from volcanic rocks dredged from different sites along the rise. In addition, isotopic compositions of rock samples from Great Meteor Seamount in the central Atlantic are presented. The new radiometric and paleontologically constrained ages identify two major episodes of volcanism: The first is the base of the rise (circa 80 to &gt;95 Ma) and the second is seamounts on the rise (0.5–16 Ma). It is proposed that interaction of the Canary hot spot with the Mid‐Atlantic spreading center formed the deep basement of the Madeira‐Tore Rise and the J‐Anomaly Ridge west of the Atlantic spreading center in the Mid‐Cretaceous. Age and geochemical data and plate tectonic reconstructions suggest, however, that the recovered Late Cretaceous volcanic rocks represent late stage volcanism from the time when the Madeira‐Tore Rise was still close to the Canary hot spot. Long after moving away from the influence of the Canary hot spot, the Madeira‐Tore Rise was overprinted by late Cenozoic volcanism. Miocene to Pleistocene volcanism at the northern end of the rise can be best explained by decompression mantle melting beneath extensional sectors of the Azores‐Gibraltar Fracture Zone (African‐Eurasian plate boundary). The geochemical compositions of these volcanic rocks suggest that the magmas were variably contaminated by enriched material within or derived by melting of enriched material underplated at the base of the lithosphere, possibly originating from the Cretaceous Canary plume. Alternatively, these late Cenozoic volcanic rocks may have derived from decompression melting of enriched pyroxenitic/eclogitic material in the upper mantle. Isotopically more depleted Pliocene to Pleistocene volcanism at the southern end of the Madeira‐Tore Rise may be related to the nearby Madeira hot spot.

Geochemistry of Late Cretaceous (60–67 Ma) igneous activities in the hebrides terrace seamount (guyot) area, Scotland

Tholeiitic basalts in various stages of alteration were dredged from Late Cretaceous volcanic rocks (60–67 Ma) in the Hebrides Terrace seamount area in the Atlantic Ocean. These rocks are extrusive olivine basalts, including high- and low-Al basalts. High-Al basalts are depleted in MgO, CaO, Cr, Sc, V, Sr, Zr and enriched in TiO2, Na2O, Nb, Rb as compared with low-Al basalts. Petrography and bulk-rock composition (major, trace and rare-earth elements) data defined clear tholeiitic suites displaying possible liquid lines of descent related to different degrees of crystal fractionation and partial melting. Isotopic dating of dredged samples gave the guyot an age of 60–67 Ma, in support of the assumption that it was formed during the Late Cretaceous.

Lithofacies and eruption ages of Late Cretaceous caldera volcanoes in the Himeji–Yamasaki district, southwest Japan: Implications for ancient large‐scale felsic arc volcanism

Abstract The Himeji–Yamasaki region in the Inner Zone of southwest Japan is underlain mainly by Late Cretaceous volcanic rocks called the Ikuno Group or the Hiromine and Aioi Groups. A new stratigraphic and geochronological study shows that the volcanic rocks in this area consist of 15 eroded caldera volcanoes between 82 and 65 Ma; they are, in order of decreasing age, the Hiromine, Hoden, Ibo, Okawachi, Seppikosan, Hayashida, Shinokubi, Fukusaki, Kurooyama, Ise, Fukadanigawa, Nagusayama, Matobayama, Yumesaki and Mineyama Formations. These calderas vary in diameter from 1 to 20 km and are bounded by steep unconformities; they coalesce and overlap each other. The individual caldera fills are composed mainly of single voluminous pyroclastic flow deposits, which are often interleaved with debris avalanche deposits and occasionally underlie lacustrine deposits. The intracaldera pyroclastic flow deposits are made up of massive, welded or non‐welded tuff breccia to lapilli tuff, and are characterized by their great thickness. The debris avalanche deposits are ill‐sorted breccia, generated by the collapse of the caldera wall toward the caldera floor during the pyroclastic‐flow eruption. The large calderas that are more than 10 km in diameter contain original values of approximately 100 km3 of intracaldera pyroclastic flow deposits. These large calderas are similar to the well‐known Valles‐type calderas in their dimensions, although it is uncertain whether their caldera floors are coherent plates or incoherent pieces. Conversely, the small calderas have diatreme‐like subsurface structures. The variety of the caldera volcanoes in this area is caused by the difference in the volume of caldera‐forming pyroclastic eruptions, as the large and small calderas coexisted. The caldera‐forming eruption rates in Late Cretaceous southwest Japan, including the studied area, were similar to those in late Cenozoic central Andes and northeast Honshu arc, Japan, but obviously smaller than those of late Cenozoic intracratonic caldera clusters in western North America and the Quaternary extensional volcanic arcs in Taupo, New Zealand. The widespread Late Cretaceous felsic igneous rocks in southwest Japan were generated by a long‐term accumulation of low‐rate granitic magmatism at the eastern margin of the Eurasian Plate.

Sunnyside: A Vertically-Preserved Porphyry Copper System, Patagonia Mountains, Arizona

Research Article| July 01, 1996 Sunnyside: A Vertically-Preserved Porphyry Copper System, Patagonia Mountains, Arizona FREDERICK T. GRAYBEAL FREDERICK T. GRAYBEAL (SEG 1976) ASARCO INCORPORATED• 180 MAIDEN LANE• NEW YORK, NY 10038 Search for other works by this author on: GSW Google Scholar SEG Discovery (1996) (26): 1–14. https://doi.org/10.5382/SEGnews.1996-26.fea Article history first online: 15 Sep 2021 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation FREDERICK T. GRAYBEAL; Sunnyside: A Vertically-Preserved Porphyry Copper System, Patagonia Mountains, Arizona. SEG Discovery 1996;; (26): 1–14. doi: https://doi.org/10.5382/SEGnews.1996-26.fea Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietySEG Discovery Search Advanced Search The Patagonia Mountains are located 65 miles south of Tucson, Arizona along the northwest-trendjng belt of porphyry copper deposits extending from Lhc Nacozari-La Caridad area in Sonora, Mexico to the Sacaton deposit in central Arizona (Figure 1). The areal geology is dominated by a large, differentiated, locally porphyritic, Laramide-age intrusion of quartz monzonite to granodiorite composilion (Figure 2) which has been dated at 58-64 my by K-Ar methods (Simons, 1974). Wall rocks are diverse, ranging from Precambrian granitic rocks to late Cretaceous volcanic rocks (Baker, 1961; Simons, 1972 and 1974).... First Page Preview Close Modal You do not currently have access to this article.

Petrology and chemistry of late Cretaceous volcanic rocks from the southernmost segment of the Western Cordillera of Colombia (South America)

This paper presents new data on the petrology and chemistry of the igneous rocks composing the Mesozoic basement of southernmost Western Cordillera of Colombia along the Ricaurte-Altaquer section. The studied sequence includes variably metamorphosed submarine lavas, breccias, tuffs and dykes of basalt to andesite composition, and minor, shallow quartz microdiorite intrusives. A Campanian age is recorded by radiolarian faunas from chert strata capping the lavas. Two different tholeiitic suites and a younger calc-alkaline suite, represented by hornblende andesite, are distinguished. One tholeiitic suite, represented by plagioclase and pyroxene phyric lavas, evolves from basalt to basaltic andesite. It is characterized by the occurrence of diopsidic pyroxene as early crystallising phase, by depletion in high-field strength elements, particularly Nb and by relative enrichment in light REE and Th. The second tholeiitic suite, which includes aphyric or poorly phyric lavas of basalt to dacite composition, differs from the first group in having moderately low FeO tot MgO ratio and lower P 2O 5 content for a given SiO 2, and higher Ti Zr and Y Zr ratios. The pyroxene chemistry of the two suites also differs. The primary geochemical characteristics of the two suites suggest a similarity with tholeiitic suites generated in island-arc environment. The hornblende andesite has mineralogical and chemical characteristics of calc-alkaline lavas erupted in an oceanic setting in an evolved island-arc. Petrologic and geochemical evidence suggests that the volcanic rocks from the Ricaurte-Altraquer section are similar to the island-arc tholeiite volcanics from the upper Macuchi Formation of northern Ecuador and can be correlated partly with this unit. Conversely, they are petrochemically dissimilar from the typical Diabase Group volcanic rocks, characterized by transitional MORB lavas, extensively present to the north in the Western Cordillera of Colombia.

Late Cretaceous paleomagnetism of the Tombstone district and vicinity: Evidence for a rotational domain boundary in southeastern Arizona

Paleomagnetic data are presented for Late Cretaceous volcanic and intrusive rocks in the Tombstone district and Dos Cabezas Mountains of southeastern Arizona. The resultant paleopole (73°N, 224°E, A95= 8°) is similar to a Late Cretaceous reference pole for North America (82°N, 210°E, A95= 7°), implying that these rocks have not been significantly rotated about vertical or near‐vertical axes relative to the continental interior. Generally, concordant results have also been published for Pennsylvanian terra rosa paleosols within the Black Prince Limestone at localities in the nearby Gunnison Hills, Whetstone Mountains, and Naco Hills. Together, however, these Carboniferous data indicate a 9°±7° clockwise rotation, with respect to North America, similar to that determined for the Colorado Plateau (5°–14°) since the mid‐Cretaceous. Published paleomagnetic data for rocks south and west of the Tombstone vicinity indicate much larger clockwise rotations ranging from ∼25° to ∼60°, implying that a rotational domain boundary runs between the Mustang Mountains and Canelo Hills to the southeast and between the Silver Bell and Tucson Mountains to the northwest. This boundary roughly coincides, in part, with the Sawmill Canyon fault zone and is parallel to other northwest trending strike‐slip faults in the region that were active during the Laramide orogeny. The clockwise rotations in southern Arizona are likely part of a larger pattern of dextral shear along the western margin of North America during the Late Cretaceous and early Tertiary caused by northward oblique subduction of oceanic plates relative to the stable craton.

Geochemistry and petrogenesis of late Cretaceous volcanic rocks from Kwangju area, South Korea.

Geochemistry and petrogenesis of late Cretaceous volcanic rocks from Kwangju area, South Korea.

Detrital modes of sand and sandstone derived from andesitic rocks as a paleoclimatic indicator

The effect of climate on the composition of volcanic sand is tested by determining detrital modes of modern stream sands derived from high-relief, andesitic rocks in areas of contrasting humid and semi-arid climates. The three humid sample areas, Mt. Pelee, Mt. Fuego, and Mt. Rainier, consist of active volcanoes and associated Neogene rocks, whereas the three semi-arid sample areas are underlain by Oligocene and Miocene (southwestern Colorado), Eocene (southwestern New Mexico), and Late Cretaceous (southwestern Montana) volcanic rocks. Point-counts of the medium sand-sized fraction of 66 samples (35 semi-arid, 31 humid) indicate that the ratio of volcanic rock fragments to plagioclase plus volcanic rock fragments (RFP index) and the ratio of volcanic rock fragments to accessory minerals (primarily monocrystalline pyroxene, amphibole, or biotite) plus volcanic rock fragments (RFA index) are statistically lower in humid regions, reflecting a greater degree of chemical breakdown of the parent rock and gravel- and sand-sized rock fragments and release of monocrystalline grains. The RFP index is applied as a paleoclimatic indicator to Late Cretaceous and Paleocene foreland-basin sandstones of the southern Foothills of Alberta, Canada. Interpretations of changes through time in paleoclimate based on the RFP index correlate closely with independent interpretations of paleoclimate based on the distribution of coal and caliche.

The occurrence of pumpellyite in basic igneous rocks from the Coloured Series NE of Neyriz, Iran

AbstractThe occurrence of pumpellyite in Late Cretaceous volcanic rocks of the Coloured Series, NE of Neyriz, is reported for the first time. The form of the pumpellyite and its chemistry are described. The mineral assemblages of the prehnite-pumpellyite facies of metamorphism are present, indicating that burial metamorphism has occurred, a new factor to be considered in the tectonic evolution of the Zagros suture zone.

Geological and petrological studies on the late Cretaceous and early Paleogene igneous rocks in central San-in District, Southwest Japan (I)

Igneous and metamorphic rocks of the central San-in district can be classified into three groups, according to their stages of activities. They are: (1) The late Cretaceous volcanic rocks resulted by volcanic activities in three different stages and accompany dykes in their intrusive facies, (2) The late Cretaceous and early Paleogene intrusive rocks resulted from the plutonic activities in two different stages, (3) The late Cretaceous and early Paleogene metamorphic intrusive rocks which were re-metamor-phosed under plutonic activities of latter stage. The late Cretaceous volcanic rocks, composed largely of rhyolite, rhyodacite and andesite, have been known to distribute mainly on the westside of the “Sanbe-Hiroshima Structural Zone” whereas the late Cretaceous and early Paleogene intrusive rocks, composed largely of diorite, quartz diorite, aplitic granite and biotite granite, occupy chiefly on the east side of this structural zone. The geological structure of basement rocks, Paleozoic in age, seems to govern the late igneous activities of this district.

Stratigraphy, Sedimentation, and Structure of Late Cretaceous Rocks in Eastern Puerto Rico--Preliminary Report

Rocks of Late Cretaceous age (Turonian to Maestrichtian) in Puerto Rico are of three types: (1) primary volcanic rocks, including tuffs, tuff breccias, and lavas; (2) intermixed pyroclastic and epiclastic rocks, including volcanic conglomerates, volcanic sandstones, and volcanic siltstones; and (3) limestones, most of which were formed as reefs around volcanic islands. These rocks, which have a maximum thickness of more than 20,000 feet, crop out along the crest and flanks of a complexly faulted, northwestward-trending anticlinorium that forms the mountainous core of Puerto Rico. In the central and south-central parts of the Commonwealth three formations are recognized. The Robles formation, which probably ranges from late Coniacian to early Campanian in age, is characterized by laminated siltstones and sandstones but includes the Lapa and Las Tetas lava members and locally at the base the rudistid-bearing Rio Maton limestone member. The Cariblanco formation, which unconformably(?) overlies the Robles formation, is probably middle Campanian to middle Maestrichtian in age. It is made up predominantly of thick coarse conglomerates separated by sandstones and siltstones. The Coamo formation of late Maestrichtian age consists chiefly of massive tuff breccias and bedded tuffs including ashflow deposits. In northeastern Puerto Rico the Upper Cretaceous section consists of the Fajardo formation, chiefly siliceous, ammonite-bearing mudstones and siltstones of Turonian and Coniacian age; interbedded sandstones, siltstones, thin limestones, pebble conglomerates, and tuff breccias that are believed to be equivalent to the Robles and Cariblanco formations of south-central Puerto Rico; and conglomerates, sandstones, and lenticular reef limestones that are believed to be equivalent to the Coamo formation. Rocks equivalent to the Fajardo formation in the central part of the island consist of massive volcanic breccias, lavas, thin tongues of sandstone, and the Aguas Buenas limestone member, a reef deposit that lies at the base of the Fajardo formation. From north-central Puerto Rico eastward the upper part of the Robles formation is represented by a thick sequence of pillow lavas and by volcanic breccia and agglomerate that appear to have accumulated along a west-northwestward-trending belt of volcanism. The strata of known Late Cretaceous age overlie massive volcanic rocks whose age has not been determined. They are conformably overlain in most places by strata probably Paleocene and Eocene in age, but are locally overlain unconformably by limestones of Oligocene age. Rocks of the Fajardo, Robles, and Cariblanco formations and their probable equivalents are chiefly subaqueous, stratified deposits that accumulated in a subsiding basin adjacent to volcanic source areas. Cross-bedding is rare in all of these rocks, but graded bedding, cyclic repetition of unsorted coarse clastics and fine-grained clastics, and slump structures indicate that turbidity currents were an important agent in the transportation and deposition of the debris. The Coamo formation, characterized by redbeds, consists chiefly of subaerial deposits including tuffs and tuff breccias. Some of the Coamo formation tuffs were transported and redeposited by mudflows or lahars, others are ashflow or hot, avalanche deposits. The present anticlinal structure of the Late Cretaceous rocks is believed to be the result of doming during intrusion of a batholith during early Tertiary time. Complex faulting that accompanied the batholithic intrusion in places helped to accentuate the anticlinal structure but in other places has modified it. Faulting has been most intensive in a belt that trends west-northwestward across the central part of the island. Normal, high-angle reverse, and transcurrent faults are recognized, but the principal fault of northern Puerto Rico appears to be a transcurrent fault of regional extent. End_Page 137------------------------------

Origin and structure of the Livingston igneous rocks, Montana

Late Cretaceous andesitic volcanic rocks immediately north of the Beartooth Mountain uplift and included in the Livingston formation originated from many small vents near the headwaters of the Deer creeks. The series is composed of volcanic breccias and tuffs near the vents and of volcanic conglomerates and sandstones farther away. The latter are interbedded with Judith River to Lance (upper Cretaceous) sediments. Pipes, dikes, and laccoliths of dacite, andesite, and diorite cut the pyroclastics and near-by sediments. Some breccia beds and the underlying sedimentaiy rocks have been tilted by the intrusion of several laccoliths prior to the Laramide orogeny. Other pyroclastics are later than the laccoliths but pre-Lance. Igneous activity had ceased before the main Beartooth thrusting.