Late Cretaceous Volcanism Research Articles

Petrogenesis of Late Cretaceous Volcanism in Kazhaba Area and its relationship with mantle plume activity of Reunion hotspot

Basaltic pillow lavas near the Kazhaba village in Balochistan are found in the tectonic slivers of the Bagh complex: the melange zone beneath the Muslim Bagh ophiolite complex. These vol-canics are mainly represented by alkali basalts. The petrography and chemistry suggest that these volcanic rocks belong to mildly to strongly alkaline, intra-plate volcanic rock series. Their low Mg# and low Cr, Ni and Co contents suggest that the parent magma of these volcanics was not directly derived from a partially melted mantle source, but resulted fractionation in an upper level magma chamber, en-route to eruption. Their LILE and HFSE, enriched primordial mantle-normalized patterns with marked positive Nb anomalies further confirm their within-plate geochemical signatures and are consistent with an enriched mantle source. Their highly enriched LREE patterns and high (La/Yb)N and (Ce/Yb)N ratios suggest a partially melted garnet-lherzolite parent magma source. The Zr versus Zr/Y studies suggest that these volcanics were derived from about 15% partially melted enriched mantle source. It is suggested that these Late Cretaceous intra-plate volcanics may represent the mantle plume activity of the Reunion hotspot, and were erupted during the passage of Ceno-Tethys Ocean floor prior to the passage of Indian Plate over it.

Removal of deep lithosphere in ancient continental collisional orogens: A case study from central Tibet, China

AbstractWidespread but small‐volume Late Cretaceous volcanic rocks in central Tibet contain important information on the Lhasa‐Qiangtang collision process. In this contribution, we focus on Late Cretaceous volcanics in the southern Qiangtang subterrane and present zircon LA‐ICP‐MS U‐Pb ages, whole‐rock major and trace element compositions, and Sr‐Nd isotopic data. Zircon LA‐ICP‐MS U‐Pb dating yielded a concordant age of 80 Ma, which postdates the Early Cretaceous collision of the Qiangtang and Lhasa terranes. The volcanic rocks are potassium‐rich alkaline andesites with high contents of K2O (3.45–5.11 wt %) and Th (13.39–25.02 ppm), as well as high K2O/Na2O ratios (0.6–0.9). They have higher REE and HFSE contents than coeval Mg‐rich and adakite‐like magmatic rocks that can be related to partial melting of a thickened lower crust. Moreover, they have higher values of Mg# and lower contents of SiO2 than lower continental crust‐derived rocks in central Tibet and experimental data of mafic rocks. We argue that the andesites were generated after the removal of thickened lithospheric mantle and subsequent to the final Lhasa‐Qiangtang amalgamation in a postcollisional setting. The high‐K characteristics can be explained by producing the primitive andesite magmas from partial melting of the residual and shallow metasomatized lithospheric mantle (the K‐rich layer) during heating by upwelling asthenosphere; subsequently, these primitive andesite magmas were subjected to fractional crystallization to generate the Amdo andesites. The way in which these andesites were formed provides evidence for the lithospheric thickening and uplifting of central Tibet during the Late Cretaceous prior to India‐Asia collision.

Gondwana breakup: no evidence for a Davie Fracture Zone offshore northern Mozambique, Tanzania and Kenya

AbstractPlate tectonic reconstructions assume a major inactive transform fault, the Davie Fracture Zone, in the West Somali Basin, along which Madagascar is thought to have migrated southwards following Gondwana breakup in the Mesozoic. Based on the interpretation of reflection seismic data, we show that the Walu Ridge offshore Kenya and the Kerimbas Basin offshore northern Mozambique are tectonically unrelated to the southward motion of Madagascar and correlate with Late Cretaceous volcanism and inversion in Kenya and the evolution of the East African Rift System respectively. Offshore Tanzania, geophysical data do not show basement structures indicating the presence of a major transform fault. These results challenge the commonly supported transform margin concept and imply a more southerly pre‐breakup position of Madagascar within Gondwana. Opening of the West Somali Basin by SW‐propagating oblique rifting and seafloor spreading is proposed.

Constraining provenance, thickness and erosion of nappes using low‐temperature thermochronology: the Northland Allochthon, New Zealand

AbstractThe Northland Allochthon, an assemblage of Cretaceous–Oligocene sedimentary rocks, was emplaced during the Late Oligocene–earliest Miocene, onto the in situ Mesozoic and early Cenozoic rocks (predominantly Late Eocene–earliest Miocene) in northwestern New Zealand. Using low‐temperature thermochronology, we investigate the sedimentary provenance, burial and erosion histories of the rocks from both the hanging and footwalls of the allochthon. In central Northland (Parua Bay), both the overlying allochthon and underlying Early Miocene autochthon yield detrital zircon and partially reset apatite fission‐track ages that were sourced from the local Jurassic terrane and perhaps Late Cretaceous volcanics; the autochthon contains, additionally, material sourced from Oligocene volcanics. Thermal history modelling indicates that the lower part of the allochthon together with the autochthon was heated to ca. 55–100°C during the Late Oligocene and Early Miocene, most likely due to the burial beneath the overlying nappe sequences. From the Mesozoic basement exposed in eastern Northland, we obtained zircon fission‐track ages tightly bracketed between 153 and 149 Ma; the apatite fission‐track ages on the other hand, generally young towards the northwest, from 129 to 20.9 Ma. Basement thermochronological ages are inverted to simulate the emplacement and later erosion of the Northland Allochthon, using a thermo‐kinematic model coupled with an inversion algorithm. The results suggest that during the Late Oligocene, the nappes in eastern Northland ranged from ca. 4–6‐km thick in the north to zero in the Auckland region (over a distance >200 km). Following the allochthon emplacement, eastern Northland was uplifted and unroofed during the Early Miocene for a period of ca. 1–6 Myr at the rate of 0.1–0.8 km/Myr, leading to rapid erosion of the nappes. Since Middle Miocene, the basement uplift ceased and the erosion of the nappes and the region as a whole slowed down (ca. 0–0.2 km/Myr), implying a decay in the tectonic activity in this region.

Geology, geochronology, and Hf isotope geochemistry of the Longtougang skarn and hydrothermal vein Cu–Zn deposit, North Wuyi area, southeastern China

The recently discovered Longtougang skarn and hydrothermal vein Cu–Zn deposit is located in the North Wuyi area, southeastern China. The intrusions in the ore district comprise several small porphyritic biotite monzonite, porphyritic monzonite, and porphyritic granite plutons and dikes. The mineralization is zoned from a lower zone of Cu-rich veins and Cu–Zn skarns to an upper zone of banded Zn–Pb mineralization in massive epidote altered rocks. The deposit is associated with skarn, potassic, epidote, greisen, siliceous, and carbonate alteration. Molybdenite from the Cu-rich veins yielded a Re–Os isochron age of 153.6±3.9Ma, which is consistent with U–Pb zircon ages of 154.0±1.3Ma for porphyritic monzonite, 154.0±0.8Ma for porphyritic biotite monzonite, and 152.0±0.8Ma for porphyritic granite. Geological observations suggest that the Cu mineralization is genetically related to the porphyritic biotite monzonite and porphyritic monzonite. All the zircons from intrusive rocks in the ore district are characterized by εHf(t) values between −13.41 and −4.38 and Hf model ages (TDM2) between 2054 and 1482Ma, reflecting magmas derived mainly from a Proterozoic crustal source. Molybdenite grains from the deposit have Re values of 14.6–27.7ppm, indicative of a mixed mantle–crust source. The porphyry–skarn abundant Cu and hydrothermal vein type Pb–Zn–Ag deposits in the North Wuyi area are related to the Late Jurassic porphyritic granites and Early Cretaceous volcanism, respectively. The Late Jurassic mineralization-related granites were derived from the crustal anatexis with some mantle input, which was triggered by asthenospheric upwelling induced by slab tearing during oblique subduction of the paleo-Pacific plate beneath the South China block, and the Early Cretaceous mineralization-related granitoids mainly from crust material formed within a series of NNE-trending basins during margin-parallel movement of the plate.

Geochemistry of metabasites of the Kolpakov Group of the Sredinny crystalline Massif in Kamchatka

Metabasites (amphibolites, garnet amphibolites, and basic crystalline schists) compose numerous sheeted bodies (often highly boudined) from a few to 100 meters thick in the plagiogneisses and migmatites of the Kolpakov Group. Chemically, they are reconstructed as basalts and picrites that were metamorphosed, as host terrigenous rocks, under the kyanite-sillimanite subfacies of the amphibolite facies (t = 620–650°C, Ps = 5.9–6.9 kbar). Metabasites are dominated by amphibolites and basic crystalline schists distributed throughout the entire section of the Kolpakov Group, whereas garnet amphibolites are more typical of the upper parts of the group, where they are intercalated with amphibolites, basic crystalline schists, plagiogneisses, and quartzites. Metaultrabasites (plagioclase-free amphibolites) occur much more rarely as small boudins up to few meters in size. According to U-Pb SHRIMP zircon dating, the plagiogneiss protolith age corresponds to the end of the Early-Late Cretaceous (90–100 Ma), which is similar in age to the weakly metamorphosed terrigenous deposits of the Kikhchik Group of the Sredinny Range. This allows us to consider the terrigenous rocks of these groups as isofacial sedimentary rocks. The same age (Early-Late Cretaceous boundary) was taken for protoliths of metabasites forming interbeds among metaterrigenous deposits of the Kolpakov Group. The interval of 100−90 Ma coincides with the beginning of the formation of the Okhotsk-Chukotka volcanogenic marginal-continental belt in East Asia. It is shown that the Kolpakov Group possesses the geochemical features of tholeiitic basalts of different geodynamic settings and comprises both typically island arc (low-Ti) and oceanic (moderate to high-Ti) tholeiites associated with ultrabasic volcanic rocks—picrites. Such a chemical peculiarity of basic rocks is typical of the marginal-continental extension zones (pull-apart basin) that were initiated on the sialic crust. It is obvious that similar geodynamic setting of the basite magmatism existed for the Sredinny Range of Kamchatka. The ascent of the mantle matter beneath the extension zone of the continental crust of the sedimentary basin and its intersection by faults that formed simultaneously with the Okhotsk-Chukotka volcanogenic belt served as the beginning of the basite volcanism in the sedimentary basin. They provided an intense fluid effect and a temperature increase in the crust with subsequent granitization and metamorphism of volcanogenic-terrigenous deposits and, finally, the development of the modern structure of the Sredinny Kamchatka Massif. The intense Late Cretaceous basite volcanism and associated granitoid magmatism in Kamchatka were presumably caused by the ascent of mantle plumes bearing hydrogen fluids.

Tomographic imaging of sub-basalt Mesozoic sediments and shallow basement geometry for hydrocarbon potential below the Deccan Volcanic Province (DVP) of India

Summary A shallow 2D tomographic P-wave velocity image is obtained in the Deccan Volcanic Province (DVP) of India using first-arrival seismic data along a 90 km long N-S trending profile in the Deccan Syneclise region. The tomographic image depicts smooth velocity variations of Quaternary (2.0-2.5 km/s) sediments, basalt (5.0-5.5 km/s) and sub-basalt Mesozoic sediments (4.0-4.5 km/s) as well as basement (5.8-6.0 km/s) configuration down to a maximum depth of 5.0 km. Due to late Cretaceous volcanism with outpouring of basaltic lava flows, this region is affected by numerous dyke intrusions and thick basaltic trap forming sills with the presence of horst and graben structures due to deep basinal faults. Although imaging below the basalt is a major challenge for the oil industry, with the help of tomographic imaging technique of long-offset seismic refraction data, we are able to image sporadic sub- and intra-trappean Mesozoic sediments (<0.75 km) deposited below thick columns (0-3 km) of basalt flows above the basement. The imaged Mesozoic sediments are expected to contain hydrocarbon due to their wide extension and suitable trapping mechanism of basaltic sills. The robustness of the velocity image is assessed through numerous tests like velocity perturbations, χ 2 estimates, RMS residuals of traveltime fit, resolution and uncertainty estimates through computation of ray-density or Hits.

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.

The Pelona-Pico Duarte basalts Formation, Central Hispaniola: an on-land section of Late Cretaceous volcanism related to the Caribbean large igneous province

Located in Central Hispaniola, the Pelona-Pico Duarte basalts Formation (Fm.) offers an opportunity to study the Late Cretaceous Caribbean large igneous province magmatism on land. It is composed by a ~2.5km-thick pile of massive and monotonous submarine flows of basalts, locally intruded by synvolcanic dikes and sills of dolerite. The Pelona-Pico Duarte basalts Fm. was emplaced onto Turonian-Lower Campanian island-arc volcanic and sedimentary sequences, and is overlain by Maastrichtian platformal carbonates. Two 40Ar/39Ar plateau ages indicate both extrusive and intrusive magmatic activity at least during the 79-68Ma interval (Middle Campanian to Maastrichtian), so the magmas were in part coeval with the late phases of the Caribbean large igneous province. The basalts have a restricted major-and trace-element, and isotopic, compositional variation. For a range of 47.6- 50.2wt.% SiO2, the Pelona-Pico Duarte basalts Fm. has relatively high contents in TiO2 (1.5-3.6wt.%) and Fe2O3T (10.7-13.1wt.%). On the basis of MgO contents, samples can be classified into tholeiitic basalts ( 20 and Zr/Nb<10) are characteristic of transitional and alkalic oceanic-island basalts. In terms of Sr-Nd isotopic composition, the samples are homogeneous and enriched relative to older Caribbean large igneous province units in Hispaniola, with (87Sr/86Sr)i ratios between 0.70330 and 0.70348 for a very restricted range of (eNd)i values between +5.0 and +5.9 (where i=70Ma). The Pelona-Pico Duarte basalts Fm. are interpreted as partial melts of a plume-related, deep enriched source, which have not been contaminated by active subduction. Mantle melt modelling indicates that both high-Mg basalts and basalts formed by mixed melts of both garnet and spinel lherzolite in variable amounts. Melts incorporated at different mantle depths, most probably in relation to the melt column processes in an upwelling plume. The Pelona-Pico Duarte basalts Fm. has significantly different values of petrogenetic tracers compared to underlying arc-related lavas, indicating a fundamental change in the mantle sources. It has geochemical affinities with the mantle domain influenced by the Late Cretaceous Caribbean plume, suggesting that enriched mantle was flowing toward the NE, to the mantle wedge region of the Caribbean island-arc, in response to rollback of the SWdirected subduction of the proto-Caribbean slab.

Geological structure of the Kashevarov Trough (central Sea of Okhotsk)

This paper reports the composition and age of rocks dredged from the Kashevarov Trough (central Sea of Okhotsk) during cruise 41 of the R/V Akademik M.A. Lavrentyev in 2006. It was found that the Late Cretaceous and Eocene volcanics from the Kashevarov Trough and Okhotsk-Chukotka volcanic belt, struc� tures of which are traceable in the Sea of Okhotsk, have similar petrographic and geochemical features. The Cenozoic sedimentary cover consists of three differentag e complexes: (1) the late Oligocene (~28.2-24.0 Ma); (2) the terminal late Oligocene-early Miocene (24.0-20.3 Ma); (3) the terminal late Pliocene-early Pleis� tocene (2.0-1.0 Ma). The upper Oligocene-lower Miocene sediments were deposited in relatively shallow� water settings, whereas the late Pliocene-early Pleistocene complex was formed in deeper environments, which was probably determined by tectonic processes. The geological data indicate that the Kashevarov Trough and the surrounding underwater rises represented in the Oligocene-early Miocene a single shelf zone of the Sea of Okhotsk, which is underlain by a structurally integral Mesozoic basement and is now subsided to depths of 800-1000 m.

Structural-dynamic relationships between the East Sikhote Alin volcano-plutonic belt and its folded basement

The East Sikhote Alin volcanoplutonic belt (ESAVB) extending along the coasts of the Sea of Japan and Tatar Strait is over 1000 km long and 35‐40 km wide on average. Similar to other elements of the East Asian volcanic belt, the latter represents a structure with the continuous development mode being superimposed or crossing protostructures of the eastern margin of the Asian continent regardless of their types and ages. The study of structural‐dynamic relationships between the basement that existed during synchronous dislocations and superimposed volcanic complexes reveals the geodynamic formation conditions of volcanic belts. The East Sikhote Alin volcano-plutonic belt was formed in the Late Cretaceous‐Paleogene. It is composed of sedimentary‐volcanogenic and volcano-plutonic rocks resting unconformably upon the folded basement represented largely by Lower Cretaceous terrigenous strata deformed into a system of open and isoclinal folds with a dominant northeastern strike (azimuth 40 ° ‐60 ° ) and complicated by imbricate updip‐ thrusts. In contrast to the basement, no dominant regional folded and dislocated systems were defined in volcanics during mapping. They demonstrate only fragments of folded‐bedded structures with variable strikes and dip angles as well as numerous fractures also oriented differently (largely, normal faults), the formation of which is thought to be related to local stresses during volcanoplutonic magmatism. Most researchers share the opinion that volcanics represents a superposed complex with its own dislocation type independent from tectonics of the Early Cretaceous folded basement. The discovery of a system of NNE-trending sinistral strikeslip faults with displacements amplitudes in the Late Cretaceous amounting to a few tens of kilometers made it possible to infer that the East Sikhote Alin volcanoplutonic belt was formed in response to Late Cretaceous activation of strike-slip faults [2‐4]. Such geodynamic conditions should result in the formation of ensembles of regularly oriented both folded structures and fractures characteristic of rocks overlying active strike-slip faults. This makes it crucial to study structures that developed in Late Cretaceous volcanics presumably in response to displacements along strike-slip faults.

Source Rock Characteristics, Thermal History, and Oil Generation of the Upper Cretaceous Island Arc Sequences from Northeastern Turkey

The Upper Cretaceous eastern Pontide island arc sequence in northeastern Turkey is composed mainly of volcanic and volcaniclastic rocks and rarely of sediments. In this study, source rock characteristics and hydrocarbon potentials of shale and claystone sequences at Zigana, Çatak, and Hacimehmet areas of the volcanic island arc were investigated. According to pyrolysis data, Zigana, Çatak, and Hacimehmet sequences show poor source rock character and they mostly contain residual organic matter and type III kerogen. In addition, shales have low potential yield and hydrogen index values. In the burial diagram drawn on the basis of Lopatin method, Σ TTI values of the Zigana, Çatak, and Hacimehmet sequences are calculated as 93,222, 1,124 and 1, respectively. This diagram indicates that Zigana and Çatak sequences are out of oil generation zone while Hacimehmet sequence has not reached oil generation zone. T max values measured from pyrolysis data generally show over mature character for the Zigana and Çatak sequences and immature character for the Hacimehmet sequence. T max values are consistent with burial diagram. Irregularities and deviations observed particularly in Zigana and Çatak sequences may be explained with thermal effect of Late Cretaceous volcanism.

Geologic implications of new zircon U‐Pb ages from the White Mountain Peak Metavolcanic Complex, eastern California

The NNW‐trending White‐Inyo Range includes intrusive and volcanic rocks on the eastern flank of the Sierran volcano‐plutonic arc. The NE‐striking, steeply SE‐dipping Barcroft reverse fault separates folded, metamorphosed Mesozoic White Mountain Peak mafic and felsic volcanic flows, volcanogenic sedimentary rocks, and minor hypabyssal plugs on the north from folded, well‐bedded Neoproterozoic‐Cambrian marble and siliciclastic strata on the south. The 163 ± 2 Ma Barcroft Granodiorite rose along this fault, and thermally recrystallized its wall rocks. However, new SHRIMP‐RG ages of magmatic zircons from three White Mountain Peak volcanogenic metasedimentary rocks and a metafelsite document stages of effusion at ∼115–120 Ma as well as at ∼155–170 Ma. The U‐Pb data confirm the interpretation by Hanson et al. (1987) that part of the metasedimentary‐metavolcanic pile was laid down after Late Jurassic intrusion of the Barcroft pluton. The Lower Cretaceous, largely volcanogenic metasedimentary section lies beneath a low‐angle thrust fault, the upper plate of which includes interlayered Late Jurassic mafic and felsic metavolcanic rocks and the roughly coeval Barcroft pluton. Late Jurassic and Early Cretaceous volcanism in this sector of the Californian continental margin, combined with earlier petrologic, structural, and geochronologic studies, indicates that there was no gap in igneous activity at this latitude of the North American continental margin.

Evidence for Late Cretaceous Volcanism in Trans‐Pecos Texas

Cretaceous basaltic pyroclastic strata have been discovered in a fault block on the southern edge of the Rosillos Mountains laccolith in the Big Bend area of Trans‐Pecos Texas. The sequence comprises base‐surge and pyroclastic‐fall deposits inferred to have accumulated on the flanks of a small phreatomagmatic volcano. A diverse assemblage of freshwater turtles (including Aspideretes), crocodile teeth, and dinosaur bones have been recovered from the uppermost part of the sequence. The fauna indicate a Late Cretaceous, probably Campanian age. An outward‐dipping normal fault bounding the pyroclastic strata on the southeast juxtaposes sediments of the Upper Cretaceous Javelina Formation in the hanging wall with pyroclastic rocks in the footwall, indicating the pyroclastic strata must be Maastrichtian or older. U‐Pb SHRIMP‐RG analyses of zircons separated from a basaltic block thrown out of the volcano yield an inferred igneous crystallization age of $$72.6\pm 1.5$$ Ma, consistent with the biostratigraphic and structural evidence. The pyroclastic strata in the fault block provide the first evidence for Late Cretaceous volcanism in the Trans‐Pecos region. Previously, the onset of igneous activity in the area was thought to be no older than 64 Ma. We speculate that the basaltic pyroclastic rocks represent an extension of the Upper Cretaceous Balcones magmatic province into Trans‐Pecos Texas.

Campanian volcanism within the Asher-1 borehole

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A Late Cretaceous pole for the Pacific plate: implications for apparent and true polar wander and the drift of hotspots

We present a Late Cretaceous (81 Ma) pole position for the Pacific plate derived from paleomagnetic analyses of basalt samples from Detroit Seamount (of the Hawaiian–Emperor seamounts) that were oriented using Brunhes-age overprints. This pole is at much higher latitudes than the previously published Late Cretaceous pole positions based on the modeling of magnetic anomalies observed during marine surveys over seamounts. Our new pole suggests that the Pacific plate would have moved rapidly between 95 and 81 Ma at speeds as high as 19.8 (−10.8/+11.2) cm/year. The Pacific plate at this time was smaller than the present-day plate and had a substantial subducting boundary. The high-velocity estimates are comparable with those of other paleoplates having similar characteristics. Therefore, plate tectonic driving forces can explain the motion and there is no need to invoke true polar wander. Decreases in mantle drag associated with vigorous Late Cretaceous volcanism in the Pacific, however, may have contributed to the rapid plate speed. The new pole position, together with other reliable paleomagnetic indicators of Pacific apparent polar wander, further supports the notion of drift of the Hawaiian hotspot during the Late Cretaceous.

Exploration Techniques at the Thorn Ag‐Au‐Cu High‐Sulfidation Epithermal Prospect, Northern British Columbia, Canada

Abstract. The Thorn prospect is an El Indio‐style high‐sulfidation epithermal prospect in a little‐explored belt of Late Cretaceous subaerial volcanics and subvolcanic intrusions in northwestern British Columbia, Canada. More than 30 massive sulfide (pyrite‐enargite‐tennantite/tetrahedrite) veins, quartz‐sulfide (quartz‐pyrite‐enargite‐tennantite/tetrahedrite) veins and quartz‐vuggy silica‐alunite veins/breccias fill ENE fractures and faults over an area of 1,600 times 1,900 m. They are hosted within a Late Cretaceous feldspar‐quartz‐biotite granodioritic porphyry stock and flanked by alteration envelopes a few meters to tens of meters wide: an inner envelope of intense sericite‐clay‐pyrite and an outer one of weak clay‐sericite‐chlorite. Where several veins are close together, the sericite‐clay‐pyrite alteration envelopes coalesce into zones up to 150 m wide.The most successful exploration techniques have been prospecting, silt and soil geochemistry, airborne magnetic and electromagnetic surveys and diamond drilling. Silt and soil geochemical anomalies for Au, Ag, As, Cu, Pb, Sb and Zn mark the known veins and indicate prospective areas where veins have not yet been discovered. Magnetics and resistivity outline the gross property‐wide structure and lithologies. Most of the sericite‐clay‐pyrite alteration and known veins are outlined by the &lt;264 ohm‐m resistivity contour, which covers an area of 800 times 2,300 m. Weak EM conductors are thought to represent undiscovered massive sulfide and quartz‐sulfide veins, and their flanking sericite‐clay‐pyrite alteration.

Stratigraphic, geochronologic, and paleomagnetic constraints on Late Cretaceous volcanism in northern Israel

Late Cretaceous volcanism in northern Israel is represented by basaltic pyroclastics and lavas at two adjacent localities at Mount Carmel and the Umm el Fahm area. Four Cenomanian and one Senonian volcanic intercalation occur in the marine succession. These volcanic rocks were dated by the 40 Ar/ 39 Ar and K‐Ar techniques using whole rock, amphibole megacrysts, and plagioclase fractions. Paleomagnetic measurements were performed on the Senonian pyroclastics. Amphibole from the oldest Cenomanian volcanic unit in Mount Carmel (the Kerem Maharal pyroclastics, at the base of the Chalk Complex) yielded an age of 97.1 ± 1.7 Ma. The penultimate Cenomanian volcanic unit in Mount Carmel (basalt at Nahal Raqefet and Muhraqa) yielded a whole-rock age of 96.7 ± 0.5 Ma. The youngest Cenomanian volcanic intercalation in the Umm el Fahm area (basalt at Me-Ammi) yielded a whole-rock age of 95.4 ± 0.5 Ma. Amphiboles from the Senonian Bat Shelomo pyroclastics, the youngest volcanic unit in the area, yielded an age of 82.0 ± 1.3 Ma. Three conclusions may be drawn from our new radiometric ages: (1) The major volcanic events operated synchronously at Mount Carmel and the Umm el Fahm area. (2) The chalk complex of Mount Carmel and the Deir Hanna Formation in the Umm el Fahm area are stratigraphically correlated. (3) The new radiometric results agree with biostratigraphically-derived ages, but provide better time constraints for the lithostratigraphic units. The data help to settle previous correlation controversies: the Albian‐Cenomanian boundary is set below the top of the Yagur Formation, and the Cenomanian‐Turonian boundary, above the base of the Muhraqa Formation.

Clay Minerals in Hydrothermally Altered Volcanic Rocks, Eastern Pontides, Turkey

AbstractExtensive hydrothermal alteration is observed around volcanogenic massive sulfide deposits. These deposits are related to Late Cretaceous volcanism in various parts of the Eastern Pontide province. Mineral assemblages resulting from alteration consist of mostly clay minerals and silica polymorphs, some sulfate minerals, and scarce zeolite minerals. The clay minerals are kaolinite, illite, and smectite. These minerals were examined using X-ray diffraction (XRD), scanning electron microscopy (SEM)-energy dispersive spectrometry (EDS), X-ray fluorescence spectroscopy (XRF), and differential thermal analysis (DTA)-thermal gravimetry (TG) techniques. The illite and the toseki deposits are a result of hydrothermal alteration of dacitic-andesitic volcanites. Two groups of bentonite deposits occur; the first mainly formed by hydrothermal solution whereas the second group resulted from halmyrolysis.The smectite in these alteration zones is generally montmorillonitic in composition and the interlayer cation is mostly Ca and lesser amounts of Na. The SiO2 and Fe2O3 contents of the hydrothermal bentonites are higher than those of the halmyrolysis smectites; however, the MgO content of both groups is similar. The Na2O and K2O contents of both groups are generally &lt;0.5%. The hydrothermal bentonites are not plastic and have open honeycomb microtextures, although the halmyrolitic smectites are plastic with ultrafine and rod-shaped textures. Illite, which contains some smectite layers, is a 1M polymorph, and has an asymmetry to the low-angle side of the XRD peaks. The impure illite deposits contain various combinations of smectite, kaolinite and gypsum, galena, sphalerite, pyrite, goetite, and quartz. The illite has &gt;35 wt. % Al2O3. The toseki raw material, which may be possibly useful as a porcelain raw material, is composed mainly of illite, kaolinite and quartz, or illite and quartz. The crystallinity of the kaolinite is poor.

Migration of the Upper Cretaceous subduction-related volcanism towards the back-arc basin of the eastern Pontide magmatic arc (NE Turkey)

The Eastern Pontide active continental margin extends along the eastern part of the Black Sea with an E–W trend and comprises the parallel oriented northern, southern and axial zones. Each zone is fault bound and distinguished by distinctive magmatism, lithofacies and stratigraphy. In the northern zone, from the coast of the Black Sea to Torul, the cessation of Liassic bimodal volcanism and accumulation of anoxic carbonates of the Valanginian–Barremian was followed by bimodal volcanism which evolved from a tholeiitic (TH)–calc-alkaline (CA) to high K–CA composition. These volcanics erupted sporadically in asymmetric deep basins until the end of the Senonian. In the southern zone, around Torul and Gümüşhane, Liassic bimodal volcanism and the accumulation of the Malm–Cenomanian platform carbonate was succeeded by Campanian volcanism, i.e. much later than volcanism in the northern zone. A second epoch of CA–high K–CA andesitic volcanism occurred in the southern zone and is intercalated with a Campanian red pelagic limestone which is also widespread in the northern zone, and known as the marker lithologic unit in the eastern Pontides. South of the southern zone, in the Bayburt–Maden area, the pelagic limestone of the Early Cretaceous and overlying ophiolitic olistostromal mélange (which contains Cenomanian MOR, WP and IA basaltic pillow lavas) are overlain by high–K calc-alkaline Campanian andesitic volcanic rocks, implying that the ophiolitic mélange was formed in a back-arc environment. The major and trace element geochemistry and REE patterns of the Upper Cretaceous volcanic rocks suggest a variation from hydrous IA to anhydrous back-arc melting conditions on the subduction zone and the migration of arc magmatism laterally from the northern to the southern zones during the Senonian. This change to dehydration melting and the migration of the subduction-related Late Cretaceous volcanism towards the south in the eastern Pontide Magmatic arc, requires a south-dipping subduction polarity during the Senonian. Copyright © 1999 John Wiley & Sons, Ltd.