Lower Cretaceous Volcanic Rocks Research Articles

Zircon U–Pb–Hf Isotopes and Whole-rock Geochemistry of Rhyolite and Tuff from the Harachiyama Formation, North Kitakami Mountains, NE Japan

The Harachiyama Formation comprises Lower Cretaceous volcanic rocks, which are distributed in the eastern margin of the Kitakami Mountains in northeastern Japan. Whole-rock chemical analysis, U–Pb zircon dating and Hf isotope analysis of the Harachiyama Formation were performed to constrain the formation age and the magmatic origin was discussed. The lava and tuff samples of the Harachiyama Formation from the Omoto and Tsukue areas supported island-arc rhyolitic rocks (SiO2 content > 70%), and yielded the weighted mean U–Pb ages of 127.8 ± 3.4 Ma and 129.2 ± 2.6 Ma (2σ), respectively. Eighteen zircon grains from two samples, dated between 141.6 Ma and 123.9 Ma, yielded positive εHf(t) values between +5.0 and +8.7. The U–Pb ages and εHf(t) values are consistent with those of the Kitakami Granitoid reported in previous studies. Therefore, it is suggested that the Early Cretaceous volcanic rocks in northeastern Japan, such as the Harachiyama Formation, have the same magmatic origin as the Kitakami Granitoid.

The Lower Cretaceous sequence of western Alaska—demise of the Koyukuk terrane?

Lower Cretaceous marine sedimentary rocks, deposited in shallow shelf and basin settings and unconformity-bound, are well exposed in southwest Alaska. Collections of Early Cretaceous fossils from across western Alaska show that similar and coeval Lower Cretaceous clastic rocks are widely distributed though only locally exposed. Volcanic rocks become an important part of the Lower Cretaceous sequence in the Yukon-Koyukuk basin, where they have been interpreted to represent a mobile intra-oceanic island arc, the Koyukuk terrane, that collided with Arctic Alaska to form the Brooks Range orogen. The volcanic rocks are chemically unlike Aleutian arc rocks but share compositional characteristics with spatially related, mid-Cretaceous alkaline intrusive rocks. The volcanic-bearing sequence was also deposited on an angular unconformity, includes both shallow shelf and basin depositional settings, and is unconformably overlain by mid-Cretaceous clastic rocks. The volcanic rocks are therefore considered part of the Lower Cretaceous sequence now identified across western Alaska. In this interpretation, the Lower Cretaceous volcanic rocks are an initial expression of the mid-Cretaceous tectonic regime that included extensional exhumation and subsidence, crustal and upper mantle melting, and high-temperature metamorphism in the hinterland of the Brooks Range orogen. The Cretaceous heating that led to hinterland crust and upper mantle change may have been caused by deep mantle disturbances in a postsubduction setting. This interpretation has implications for the timing of contractional orogenesis, the location and nature of the related continental borderland, and the tectonic setting for development of the Anguyucham and related oceanic terranes.

Sinistral displacement along the Tan–Lu Fault during the Cretaceous induced by Paleo-Pacific subduction: Constraints from new paleomagnetic and U–Pb geochronological data

The Jiaodong Peninsula in eastern China is separated from the main mass of the North China Craton by the Tancheng–Lujiang (Tan–Lu) Fault, the largest fault system in eastern Asia. Although accumulating lines of evidence have suggested that the Tan–Lu Fault underwent strike-slip movement in the Mesozoic, there is still debate about the timing and magnitude of the displacement. To decipher this problem, we conducted paleomagnetic and U–Pb geochronological studies of the Lower Cretaceous volcanic rocks of the Qingshan Group on the Jiaodong Peninsula, to the east of the Tan–Lu Fault on the southeastern edge of the North China Craton. LA–ICP–MS U–Pb dating of zircons from five samples yielded consistent ages of ca. 120 Ma. Stepwise thermal demagnetization conducted on 308 samples (40 sites) from two sections revealed stable characteristic remanent magnetization directions. After paleomagnetic tests and rejecting inappropriate data, the mean direction from 288 samples (31 independent units) is 200.3°/–51.4° (α95 = 4.1°), with a corresponding pole at 215.1°/73.0° (A95 = 4.8°). The paleolatitude calculated for the Jiaodong Peninsula is lower than that of the main block of the North China Craton, which indicates that latitudinal offset occurred between these two parts, and this offset can be ascribed to strike-slip movement along the Tan–Lu Fault. Our data provide new constraints on regional tectonic movement and Paleo-Pacific subduction processes during the late Mesozoic.

Natural fractures in tight gas volcanic reservoirs and their influences on production in the Xujiaweizi depression, Songliao Basin, China

Lower Cretaceous volcanic rocks that contain major natural fractures are the main natural gas reservoirs in the deep Xujiaweizi depression of the northern Songliao Basin in China. Natural fractures are mainly tectonic fractures, in addition to some shrinkage, dissolution, and cryptoexplosive fractures. Analyzing various sources of data revealed that fractures are generally better developed in the rhyolites and trachytes of the crater and near-crater group. Also, tectonic fractures are developed to a higher degree in the hanging wall of faults, with the degree of development decreasing with increasing distance from the fault planes. More than 85% of these fractures are found to be unfilled and, when connected by pores, provide the main effective storage volume for these volcanic reservoirs (up to ∼75%). Natural gas production is closely related to the linear density (P10) of unfilled tectonic fractures and the angle between the orientation of unfilled tectonic fracture and the corresponding direction of the maximum horizontal stress component. The results show that natural gas production is elevated when the average P10 exceeds 2 m−1, and the average included angle is less than 30°. Finally, it was concluded that within the study area, unfilled tectonic fractures of near east-west strikes, which have smaller included angles (<30°) with respect to the maximum horizontal stress direction, are under minimal compressive stresses and make the best fluid-flow pathways in the reservoir.

Geochemical and Stratigraphic Analysis of the Chisana Formation, Wrangellia Terrane, Eastern Alaska: Insights Into Early Cretaceous Magmatism and Tectonics Along the Northern Cordilleran Margin

AbstractThe Chisana Formation consists of Lower Cretaceous volcanic rocks that occur in the Nutzotin Mountains of eastern Alaska. New stratigraphic analysis indicates that the volcanic succession is &gt;2 km thick at the Bonanza Creek type section. We present stratigraphic, geochemical, Sr‐Nd‐Pb isotope, and U‐Pb age data from samples collected from various stratigraphic levels of the Chisana Formation. We demonstrate that the Chisana Formation can be divided into a lower subaqueous unit, a middle transitional unit, and an upper subaerial unit. Chisana Formation lavas range from transitional to subalkaline basalts through andesites. Trace element geochemistry shows high field strength element depletions relative to large ion lithophile elements and hydrous mineral assemblages with calc‐alkaline to tholeiitic chemistries, all consistent with a magmatic arc origin. Chisana lavas yield geochemical compositions and isotope characteristics that overlap with magmas from volcanic suites formed within juvenile continental crust and immature island arcs. Volcanism occurred between ~131 and 117 Ma judging from previously reported lava ages and new U‐Pb ages of detrital zircons recovered from sandstones that conformably underlie the lowermost Chisana Formation lavas. Our results support existing tectonic models in which an east dipping subduction zone existed beneath Wrangellia during Early Cretaceous time. The upsection shift from marine to terrestrial depositional conditions in the Chisana Formation and the overlying ~117–93 Ma Beaver Lake Formation was coincident with regional shortening. Together, the geologic evidence for shortening and terrestrial deposition are interpreted to reflect accretion/suturing of Wrangellia against inboard terranes.

Palaeomagnetic assessment of tectonic rotation in Northeast Asia：implications for the coupling of intracontinental deformation and mantle convection

ABSTRACT Knowledge of the displacement history of the series of continental fragments in Northeast Asia is key to understanding the plate kinematic evolution of the western peri-Pacific tectonic domain. Palaeomagnetism is an effective approach for deciphering the horizontal movements of crustal elements, especially vertical-axis rotations. Here we present a set of new palaeomagnetic data from Lower Cretaceous volcanic rocks from the northeastern part of the North China Block (NCB), and from Lower Cretaceous sedimentary and volcaniclastic rocks from the Bureya-Jiamusi-Khanka terrane. The data suggest that the northeastern NCB crustal elements underwent a clockwise motion relative to the Eurasian continent after the Early Cretaceous, and in addition a substantial counterclockwise deformation component is observed in the Bureya-Jiamusi-Khanka terrane. Combined with the results of previous Cretaceous and Cenozoic palaeomagnetic studies, we suggest that the northeast part of the North China Block and the Korean Peninsula experienced ~20° of clockwise rotation during the Cenozoic. However, the Bureya-Jiamusi-Khanka terrane and its eastern neighbouring Pacific-related accretionary terranes, such as the Sikhote-Alin Orogenic belt, however, underwent a contemporaneous 40°counter-clockwise rotation. The Japanese Islands, with juvenile crust in the easternmost part of Asia, were divided into a pair of terranes, which rotated in opposite directions during the Middle Miocene. The opposing senses of rotation are attributed to mantle wedge convection. We suggest that tectonic rotations of parts of Northeast Asia are controlled by the subduction of the western Pacific Plate via lithospheric deep-shallow coupling. The two critical events were coeval with ridge subduction of the Izanagi-Pacific mid-ocean ridge under the continental margin during the Palaeocene, and the collision of the Izu-Bonin and Japanese Islands during the Middle Miocene, respectively.

Petrogenesis and tectonic setting of Early Cretaceous magmatism in the Jiwa area, central Lhasa Terrane, Tibet

ABSTRACTNew zircon LA-ICP-MS U–Pb ages, Sr-Nd isotopic data, and whole-rock major and trace element data from Early Cretaceous volcanic rocks are reported for the Jiwa area in the southern central Lhasa Terrane of Tibet. These mainly silicic volcanic rocks and subordinate intermediate-basic volcanic rocks have long been considered to be Pliocene (Wuyu Group) or Eocene (Pana Formation) in age. However, our new zircon U–Pb ages constrain the timing of eruption to the Early Cretaceous (124.6 ± 1.6–126.1 ± 1.1 Ma); thus, we have redefined these volcanic rocks as the Lower Cretaceous Zenong Group. The silicic volcanic rocks feature high-K calc-alkaline to shoshonitic compositions and are mostly strongly peraluminous, rich in Rb, Th, and light rare earth elements (LREEs), and depleted in Nb, Ta, P, and Ti. They are also characterized by negative whole-rock εNd(t) values (–9.1 to –13.1) and variable 87Sr/86Sr ratios. Thus, the geochemical and zircon U–Pb age data of the Jiwa volcanic rocks suggest that these rocks are associated with a continental arc and are mostly likely derived from anatexis of ancient continental crustal material and minor basalt-derived melts. The discovery of Early Cretaceous volcanic rocks in the southern central Lhasa Terrane extends the duration of magmatism triggered by southward subduction of the Bangong-Nujiang oceanic lithosphere from the Late Jurassic to the Early Cretaceous. The spatial distribution of magmatism is also extended 70–80 km to the south. The Lower Cretaceous volcanic rocks in the Jiwa area are proposed to be a result of bi-directional subduction, with southward subduction of the Bangong-Nujiang oceanic crust and northward subduction of the Yarlung Zangbo oceanic crust. The bi-directional subduction of the oceanic lithosphere and gravitational sinking led to slab retreat at ca. 125 Ma. The roll-back of the slab would have then led to back-arc extension and asthenospheric upwelling. The subduction-induced decompression melting of the mantle led to the generation of widespread rhyolitic volcanism with continental arc geochemical signatures.

A syn-collisional model for Early Cretaceous magmatism in the northern and central Lhasa subterranes

Voluminous Early Cretaceous volcanic rocks from the northern and central Lhasa subterranes contain important information on the tectono-magmatic processes. In this contribution, we focus on the Lower Cretaceous volcanic rocks in the Nagqu area, northern Lhasa subterrane, and present their zircon LA-ICP-MS U–Pb ages, in situ Hf isotopic data, whole-rock major and trace element compositions, and Sr–Nd isotopic data. The Nagqu volcanic rocks are high-K calc-alkaline to shoshonitic (K2O: 1.41–8.52wt.%; K2O/Na2O: 0.36–6.65) and feature similar geochemical characteristics (e.g., subparallel distribution of incompatible elements and REEs). Assimilation–fractional crystallization of magmas from identical source (rather than diverse magma sources) was mainly responsible for the formation of the diverse volcanic rock types. High Hf/Sm (>0.7) and high U/Yb whole-rock values and low Y values in zircon grains suggest the involvement of terrigenous components rather than subducted oceanic crust in the magma source. Tectonic discrimination diagrams, sedimentary environment (marine–continental transition), magma compositions (K-rich and terrigenous components), crustal thickening, and spatio-temporal variations in subduction, syn-collisional and post-collisional processes, indicate that the geodynamic setting of the Early Cretaceous magmatism was associated with the collision between the Lhasa and Qiangtang terranes. The high-K characteristics were inherited from the melts derived from the partial melting of lower metasomatized lithospheric mantle (the K-rich layer), which was transported to great depths by the continuously thickening lithosphere, eventually triggering melting.

Late Cretaceous evolution of the Coqen Basin (Lhasa terrane) and implications for early topographic growth on the Tibetan Plateau

The tectonic evolution of the Lhasa terrane (southern Tibetan Plateau) played a fundamental role in the formation of the Tibetan Plateau. However, many uncertainties remain with regard to the tectonic and paleogeographic evolution of the Lhasa terrane prior to the India-Asia collision. To determine the early tectonic processes that controlled the topographic evolution of the Lhasa terrane, we analyze the Cretaceous strata exposed in the Coqen Basin (northern Lhasa subterrane), which comprises the Langshan and Daxiong Formations. The Langshan Formation unconformably overlies the volcanic rocks of the Lower Cretaceous Zelong Group and consists of similar to 80 m of Orbitolina-bearing limestones, which were deposited in a low-energy, shallow marine environment. Micropaleontological analysis indicates that the Langshan Formation in the Coqen Basin was deposited from late Aptian to early Cenomanian times (ca. 113-96 Ma). The overlying Daxiong Formation (similar to 1700 m thick) consists of conglomerate, coarse sandstone, and siltstone with interbedded mudstone, and represents deposits of alluvial fans and braided rivers. The Daxiong Formation was deposited after the early Cenomanian (ca. 96 Ma) and accumulated until at least ca. 91 Ma, indicating accumulation rates of greater than 0.3 km m.y.(-1). By combining paleocurrent data, sandstone petrology, detrital zircon U-Pb ages, and Hf isotope analysis, we demonstrate that the Daxiong Formation was derived from Lower Cretaceous volcanic rocks and pre-Cretaceous strata in the northern Lhasa subterrane. During Late Cretaceous time, two thrust systems with opposite vergence were responsible for transforming the northern Lhasa subterrane into an elevated mountain range. This process resulted in the evolution from a shallow marine environment (Langshan Formation) into a terrestrial depositional environment (Daxiong Formation) on the southern margin of the northern Lhasa subterrane. Given the regional paleogeographic context, we conclude that the Daxiong Formation in the Coqen Basin records local crustal shortening and flexure resulting in foreland basin development on the southern margin of the northern Lhasa subterrane, which implies early topographic growth of the northern Lhasa subterrane in southern Tibet prior to the India-Asia collision.

Tectonic significance of Cretaceous-Tertiary magmatic and structural evolution of the northern margin of the Xolapa Complex, Tierra Colorada area, southern Mexico

The Tierra Colorada area sits along the northern limit of the Xolapa Complex, where it is juxtaposed against the Mixteco (Paleozoic) and Guerrero (Mesozoic) terranes of southern Mexico, just north of Acapulco. This paper presents combined structural and geochronological data from Tierra Colorada area that show evidence of four deformational events and several episodes of arc magmatism during Mesozoic and Cenozoic time. The oldest magmatism is represented by ca. 165 Ma granitoids and was followed by intrusion of the foliated El Pozuelo granite (129 ± 0.5 Ma; concordant U-Pb zircon analysis). This intrusion postdates D 1 metamorphism and migmatization in the Xolapa Complex. The next magmatic episode is represented by the peraluminous, foliated El Salitre granite (55.3 ± 3.3 Ma; mineral–whole-rock Rb-Sr isochron) and the protomylonitic Las Pinas I-type granite (54.2 ± 5.8 Ma; lower intercept U-Pb zircon). Las Pinas granite is characterized by D 2 ductile fabric with normal, top-tothe north-northwest sense of shear, deformed at 45–50 Ma (Rb-Sr and K-Ar ages). The ca. 34 Ma undeformed granites correspond to the last intrusive pulse in the area, postdating both D 3 south-southwest–verging thrusting of the Cretaceous Morelos Formation over sheared granites and Lower Cretaceous volcanic rocks, and open folding during D 4 . These four pulses of subduction-related magmatism in the Tierra Colorada area indicate a regular northeastward subduction at the Mesoamerican trench since Jurassic time, and alternate with contractile and/or extensional tectonic events. The gap in magmatic activity ca. 90–100 Ma roughly coincides with deposition of platformal limestones of the Morelos Formation during the middle Cretaceous. The stable conditions during deposition of the Morelos Formation may have resulted from a combination of backarc extension and development of a passive margin during the Early–middle Cretaceous, which postdated the accretion of an exotic block, either the Guerrero terrane or the Chortis block. Following the Laramide orogeny in southern Mexico (roughly during the Late Cretaceous) the Paleocene–Miocene tectonic evolution in the Tierra Colorada area involved an alternation of magmatic pulses with extensional and contractile events. This was the result of a combination of several factors, including the geometry of the subducted slab, convergence rate, stress transmission between the subducting and overlying plates, and the rate of subduction erosion.

Cretaceous accretionary complex related to Okhotsk-Chukotka Subduction, Omgon Range, Western Kamchatka, Russian Far East

The Omgon Range of Western Kamchatka contains a mid to Upper Cretaceous sequence of flysch with tectonic inclusions of Jurassic–Cretaceous oceanic rocks inferred to have been imbricated together in an accretionary prism. These rocks were tectonically juxtaposed during the Cretaceous in a mélange that contains a number of elements but mainly includes: (1) Middle Jurassic–Lower Cretaceous volcanic rocks formed in an oceanic and/or marginal sea environment; and (2) Albian–Campanian terrigenous turbidites made of quartz-rich clastic sediments that accumulated near a continental-margin. The oceanic rocks are inferred to have been tectonically incorporated into the continental terrigenous unit by offscraping during subduction. The accretionary prism resulted from subduction of the Pacific paleo-oceanic plate (Izanagi) under the Eurasian continental margin, which ultimately caused volcanism in the inboard Okhotsk-Chukotka volcanic belt. Internal imbrication was completed by the Maastrichtian (∼70 Ma) as indicated by apatite fission-track ages that record cooling and exhumation of this crustal block. The Omgon accretionary wedge originated in a similar geodynamic setting and same time as the Yanranai (northern Korayk), Tonino–Aniva (southeastern Sakhalin), Hidaka (northeastern Japan) and Cretaceous part of the Shimanto belt (southwestern Japan). The similarities of ages, lithology, and tectonic setting suggest that the Omgon accretionary wedge was part of a paleo-subduction zone along the Eurasian margin during the mid to Late Cretaceous.

Cordilleran-margin quartzites in Baja California – implications for tectonic transport

One of the current controversies in Cordilleran tectonics concerns the position of Baja California prior to ∼300 km of opening of the Gulf of California. Geologic arguments, together with paleomagnetic results from Lower Cretaceous volcanic rocks, suggest that the rocks of the Baja Peninsula formed and evolved along the coast of northwestern Mexico prior to opening of the Gulf. In contrast, paleomagnetic data from Cretaceous–early Tertiary plutonic rocks and clastic strata have been interpreted by some workers to suggest that Baja was located near southern Mexico at approximately 80 Ma. The presence of similar detrital zircon ages in lower Paleozoic quartzites of northeast Baja and in lower Paleozoic strata east of the Gulf, in northwestern Mexico and southwestern US, provides strong support for the northern paleoposition, suggesting that Baja has been transported northward by only ∼300 km.

Mineralogy, Geochemistry, and Age Constraints on the Zn-Pb Skarn Deposit of Maria Cristina, Quebrada Galena, Northern Chile

The Maria Cristina Zn-Pb skarn deposit, hosted by carbonate rocks of the Lower Cretaceous Chanarcillo Group in northern Chile, is described to provide a basis of comparison to adjacent (~2 km) barite deposits and regional Pb-Zn and Ba deposits with Mississippi Valley-type affinities. Strongly retrograded garnet (andradite to Ad30Gr70), diopsidic pyroxene, and epidote skarn, and barite-bearing massive sulfides (sphalerite-pyrite-galena-marcasite-magnetite) occur at the contact of potassically altered diorite porphyry of mid-Cretaceous age. The skarn mineral compositions, similar to those of Cu skarns, may reflect emplacement in a high f O2 context. Sulfur isotope values of sulfides (–8.8 to +7.1‰) and barite (14.0–26.1‰) indicate disequilibrium conditions and different sulfur sources, including magmatic sulfur. Lead isotope ratios of galena and strontium isotope ratios of barite indicate a similar metal reservoir for the different ore deposit types, constituted by the intercalated Lower Cretaceous volcanic rocks or mid-Cretaceous intrusive rocks. 40Ar/39Ar dating of hydrothermal K feldspar suggests an age between 94 and 90 Ma for the mineralization.

Coastal and Baja California paleomagnetism reconsidered

Systematic reappraisal of paleomagnetic data from upper Mesozoic and lower Cenozoic sedimentary rocks and igneous rocks of late Mesozoic age in coastal and Baja California indicates that pluton tilt and shallowing of paleomagnetic directions by sedimentary compaction resolve observed discordances with the North American apparent polar wander (APW) path when the best available reference paleopoles are used for comparison. The hypothesis of major tectonic transport in excess of distances inherent in current models for evolution of the San Andreas transform system is unnecessary and in conflict with geologic observations. Fresh insights derived from recent data sets and improved analysis include the following. (1) The limited paleomagnetic data recording primary remanence for Upper Jurassic and Lower Cretaceous volcanic rocks in coastal California are concordant with the North American APW path. (2) The tilt of the Peninsular Ranges batholith inferred from paleobarometric data eliminates its declination discordance and reduces its inclination discordance to a level where ovals of confidence for observed and expected paleomagnetic directions overlap. (3) Key Cretaceous and Paleogene red-bed successions on the Salinian block of coastal California and in Baja California have yielded concordant paleolatitudes. (4) The discordances of different apparent paleolatitudes inferred from Cretaceous and Paleogene marine sedimentary strata lie within a range that can be attributed to compaction shallowing on the basis of both experimental information and observations from elsewhere. (5) Turbidites are more susceptible to compaction shallowing than shelf and deltaic strata. (6) The uniform rotation of paleomagnetic directions inferred previously for peninsular California is not supported by the full array of data now available from multiple localities. (7) Basement ties of the Salinian block in California and of the Baja California peninsula to the continental interior preclude any hypothesis for tectonic transport much in excess of San Andreas transform slip.

Igneous history of the Koyukuk Terrane, western Alaska: Constraints on the origin, evolution, and ultimate collision of an accreted island arc terrane

The Koyukuk terrane of western Alaska consists of volcanic, volcaniclastic, and plutonic rocks which range from Late Paleozoic to Early Cretaceous in age. The terrane crops out in a U‐shaped belt which is roughly paralleled by outer belts of ultramafic rocks, oceanic plate basalts and cherts, and retrograded blueschist facies rocks of continental protolith. These rocks have been interpreted as components of a volcanic arc terrane that collided with the North American continental margin in Early Cretaceous time. The Koyukuk terrane consists of four time‐stratigraphic units: (1) pre‐Middle Jurassic basalts, (2) Middle and Late Jurassic granitic rocks, (3) lower Lower Cretaceous volcanic rocks, and (4) upper Lower Cretaceous volcanic rocks. Limited chemical data from the basalts of unit 1 indicate that they were erupted in a nonarc tectonic environment, possibly in an oceanic island or back arc setting. Units 2, 3, and 4 have the characteristics of subduction‐related volcanic rocks (i.e., depleted Nb and Ta and enriched alkaline elements, relative to the light rare earth elements). Unit 3 contains tholeiitic, calc‐alkaline, and alkaline rocks with chondrite‐normalized rare earth element patterns that range from flat (LaN/YbN = 1) to highly light rare earth element enriched (LaN/YbN &gt; 15). The highly alkaline or shoshonitic lavas were erupted toward the end of unit 3 time (Valanginian) during the final stages of arc‐continent collision. These alkaline lavas could have been derived by very small degrees of partial melting of a similar source to that of the earlier arc lavas. Unit 4 lavas are also alkaline or shoshonitic, but their incompatible element composition indicates that they were derived from a different source than that of the earlier arc lavas. These late alkaline lavas are chemically similar to crosscutting mid‐Cretaceous plutons whose isotopic compositions (Arth et al., this issue (a)) suggest derivation by partial melting of distinctly older subcontinental lithosphere. We speculate that the parental magmas of unit 4 lavas may also have been derived by partial melting of this subcontinental mantle which was underthrust beneath the Koyukuk arc terrane during the final stage of arc‐continent collision.

Granitic rocks closely associated with the Lower Cretaceous volcanic rocks in the Kitakami Mountains, Northeast Japan.

Granitic rocks closely associated with the Lower Cretaceous volcanic rocks in the Kitakami Mountains, Northeast Japan.