Volcano-plutonic Arc Research Articles

Tectonic Significance of Polymodal Compositions in Melange Sandstones, Western Melange Belt, North Cascade Range, Washington

ABSTRACT Upper Jurassic to Lower Cretaceous sandstones of the western melange belt in the North Cascade Range of Washington contain three distinct petrofacies of arkosic, lithic, and chert-rich compositions. These petrofacies were most likely mixed together in an accretionary wedge within an active subduction zone. The arkosic and lithic petrofacies were probably derived from a differentially dissected volcano-plutonic arc system, and the chert-rich sandstones were in part derived from reworking of uplifted ocean-floor deposits in the developing accretionary wedge. Compositional similarity between the western melange belt and the contemporaneous Franciscan Complex of northern California suggests that similar sedimentary-tectonic processes operated in the two melange systems, or that similar po ymodal compositions can result from more than one process, or that the two units, now widely separated, were once together.

Cathaysia, Gondwanaland, and the Paleotethys in the evolution of continental Southeast Asia

Continental Southeast A ia is dominated by Precambri an continenral blocks overlain by Late Proterozoic to Paleozoic platform successions, representing Atl an ti c-type rifted miogeocl in al margin s. All th e blocks appear to have rifted and drifted from the Australian part of Gondwanaland. The timing and ex tent of their eparati on is ana lysed by th e distribution of Penni an Cathays ian Gigamop­ leris and Gondwana Glossop1eris floras, assisted by dated tectono-stru ctura l units, pa leoclimate ind icators, and good quality paleomagnetic data. Between th e blocks lie narrow intensely folded Ph anerozoic mobile belts, which developed on the oceanic crust of th e Paleoteth ys ocean, c haracteri zed by pela gic-turbidite flysch equences which shallowed as th e oceans narrowed. The narrowing was effected by subduction resultin g in island arcs within the oceans, and cordill eran volcano-plutonic arcs along the bl ock marg in . Extinction of th e bas in s resulted in colli s ion zones containin g S-type granites and utu re zones containin g dismembered ophi olites. Post-consolid ation pl ate readju tments res ulted in wrench and rift fa ulting in several places while convergence conti nu ed elsewhere. The tectoni c analysis has been carried out by recognizing tectonic elements (s tructural-formati onal unit~) for selected Phanerozoic time frame . We also pre ent a Phanerozoic sequence of palinspatic reconstructiors fo r the ri fti ng and drift in g of the blocks from northern Australia . These are consistent with the known ages and distribution s of th e tectono-structural units a nd th e good quality paleomagnetic data. We also present an ana lys is of the first-order re lat ionship between the di tributi on of mineral deposits and the igneous events which resulted from tectonic reactions between the Precambrian blocks and th e mobile be lt s.

Ore-related breccias in volcanoplutonic arcs

An overview of breccias related to a variety of base metal, precious metal, and lithophile element deposits in volcanoplutonic arcs permits definition of six possible mechanisms for subsurface brecciation.1. Release of magmatic-hydrothermal fluids from high-level hydrous magma chambers during second boiling and subsequent decompression generates a spectrum of breccia types in which fragments may suffer collapse and/or ascent. Single or multiple intrusion-related breccia pipes and pre- and intermineral breccias in porphyry copper deposits provide widespread examples.2. Magmatic heating and expansion of meteoric pore fluids may lead to brecciation, commonly of late or postmineral age and including pebble dikes, in porphyry-type and related deposits. Magmatic heating of rocks saturated with seawater may cause submarine hydrothermal eruptions late in the emplacement histories of Kuroko-type massive sulfide deposits; many of the resultant breccias underwent limited sedimentary transport. Overpressuring of heated fluids beneath semipermeable, partly self-sealed cap rocks may lead to brecciation and subaerial hydrothermal eruptions in shallow epithermal precious metal settings; magmatic heating or tectonic disturbance may have triggered brecciation.3. Interaction of cool ground waters with subsurface magma can generate phreatomagmatic explosions. Postmineral phreatomagmatic diatremes associated with porphyry systems and premineral diatremes with epithermal precious (+ or - base) metal deposits were generated in this manner; these attained the palcosurface to produce pyroclastic base surge and fall deposits that accumulated as tuff rings around maar craters.4. Magmatic-hydrothermal brecciation may lead to disruption of rocks through to the paleosurface, decompression, and fragmentation and eruption of the top part of an underlying magma chamber. Pre- and postmineral magmatic diatremes of this sort are inferred to accompany a few porphyry-type and other base and precious metal systems; they were manifested at the palcosurface by accumulations of pyroclastic fall and flow deposits.5. Breccias may result from mechanical disruption of wall rocks during subsurface movement of magma. Any intrusion-related deposit may include such intrusion breccias.6. Tectonic breccias resulting from fault displacement may accompany any type of ore deposit.A continuum exists between many of these breccia types and it is difficult to identify unique criteria for their unambiguous distinction.

Origin of the Torlesse terrane and coeval rocks, South Island, New Zealand

The Carboniferous to Lower Cretaceous Torlesse terrane and Haast Schist derivatives constitute the major part of the complexly deformed facies of the Eastern Province of New Zealand. Strata consist mainly of quartzofeldspathic graywacke and mudstone, intercalated with minor but widely distributed conglomerate, and volcanics with associated chert and limestone. Clastic rocks were deposited largely by sediment gravity-flow mechanisms in a deep-marine environment. Also present are a few highly fossiliferous shallow-marine and terrestrial deposits of limited areal extent, which rest unconformably on or in fault contact with Torlesse flysch. Several periods of deformation are recognized and melange is present on both local and regional scales. Metamorphism ranges from zeolite to greenschist facies. The bulk of the rocks fall into five areally extensive and mutually exclusive fossil zones of the following ages: Permian ( Atomodesma ), Middle Triassic, early Late Triassic (?) ( Torlessia ), Late Triassic ( Monotis ), and Late Jurassic-Early Cretaceous. Contacts between major fossil zones are mainly tectonic. Petrographic analysis permits subdivision of Torlesse sandstones into five major petrofacies that correspond in age to the five major fossil zones. The sandstone petrofacies (mainly arkosic), together with the composition of conglomerate clasts (mainly indurated Torlesse rocks), indicate that the source terrane was a continental volcano-plutonic arc, probably part of Gondwanaland, coupled with autocannibalistic reworking of older uplifted Torlesse rocks. In contrast to the quartzofeldspathic nature of the Torlesse, coeval sedimentary rocks of the Eastern Province are volcanogenic. They are thought to represent related forearc-basin (Maitai-Murihiku terranes) and trench-complex (Caples terrane) deposits derived from a volcanic island arc (Brook Street terrane). Three petrofacies are established for Maitai-Murihiku and Caples sandstones. The petrofacies indicate a common, evolving, immature to submature volcanic island arc source for these terranes. A reconstruction of New Zealand's Eastern and Western Provinces is proposed. In Permian and Triassic times, the Torlesse was deposited in trench, slope, or borderland basins along a trench-transform margin fronting a continental volcano-plutonic arc source (Western Province-Gondwanaland). Deposition was spasmodic but voluminous and was accompanied by concurrent deformation and accretion resulting in parallel belts of Torlesse rock younging outward from the Gondwanaland margin. At the same time, the Brook Street terrane volcanic arc and associated terranes were forming to the west of the Torlesse site, separated from Gondwanaland by a marginal sea. In latest Triassic or Early Jurassic times, the Torlesse was rafted into the volcanic arc system via transform faulting approximately parallel to the Gondwana margin. The collision event resulted in tectonic thickening of Torlesse and Caples rocks at the plate interface and metamorphism to Haast Schist. The source was then dominated by older, partly metamorphosed Torlesse terrane, newly uplifted along the collision front. Closing of the marginal sea behind the Brook Street terrane in Late Jurassic-Early Cretaceous times resulted in juxtapositioning with the Western Province (Gondwanaland) along the Median Tectonic Line.

Classification and genesis of stockwork molybdenum deposits

Classification and genesis of stockwork molybdenum deposits

K-Ar geochronology of the late cenozoic volcanic rocks of the Cordillera Occidental, southernmost Peru

Twenty-four K-Ar radiometric ages are presented for late Cenozoic continental volcanic rocks of the Cordillera Occidental of southernmost Perú (lat. 16° 57′–17° 36′S). Rhyodacitic ignimbrite eruptions began in this transect during the Late Oligocene and continued episodically through the Miocene. The development of andesitic-dacitic strato volcanoes was initiated in the Pliocene and continues to the present. The earliest ignimbrite flows (25.3–22.7 Ma) are intercalated in the upper, coarsely-elastic member of the Moquegua Formation and demonstrate that this sedimentary unit accumulated in a trough, parallel to Andean tectonic trends, largely in the Oligocene. More voluminous ash-flow eruptions prevailed in the Early Miocene (22.8–17.6 Ma) and formed the extensively preserved Huaylillas Formation. This episode was coeval with a major phase of Andean uplift, and the pyroclastics overlie an erosional surface of regional extent incised into a Paleogene volcano-plutonic arc terrain. An age span of 14.2–8.9 Ma (mid-Late Miocene) is indicated for the younger Chuntacala Formation, which again comprises felsic ignimbrite flows, largely restricted to valleys incised into the pre-Huaylillas Formation lithologies, and, at lower altitudes, an extensive aggradational elastic facies. The youngest areally extensive ignimbrites, constituting the Sencca Formation, were extruded during the Late Miocene. In the earliest Pliocene, the ignimbrites were succeeded by more voluminous calcalkaline, intermediate flows which generated numerous large and small stratovolcanoes; these range in age from 5.3 to 1.6 Ma. Present-day, or Holocene, volcanism is restricted to several large stratovolcanoes which had begun their development during the Pleistocene (by 0.7 Ma). The late Oligocene/Early Miocene (ca. 22–23 Ma) reactivation of the volcanic arc coincided with a comparable increase in magmatic activity throughout much of the Cordilleras Occidental and Oriental of the Central Andes.

Evolution of Sedimentary Systems During Mesozoic and Cenozoic in Southern Alaska--An Overview: ABSTRACT

The evolution of sedimentary systems during the Mesozoic and Cenozoic in southern Alaska can be divided into six episodes, each representing a tectonic cycle that is presently recorded, with varying degrees of clarity, as a plutonic belt containing related sedimentary rocks that are bounded by unconformities or faults. These six episodes or tectonic cycles, which are sequential but sometimes overlap, occurred during the: (1) Permian(?) through Early Jurassic, (2) Early Jurassic into Early Cretaceous, (3) late Early and early Late(?) Cretaceous, (4) Late Cretaceous, (5) early Cenozoic, and (6) late Cenozoic. Each tectonic cycle began with magmatism and volcanism that constructed a magmatic arc with a back-arc and fore-arc basin, outer-arc ridge, shelf, slope, trench, and abyssal plain. Uplift and erosion dissected the magmatic arc and outer-arc ridge, and sediment filled the arc basins, shelf, slope, and trench, and then spilled onto the abyssal plain. Throughout the cycle, tectonism, magmatism, and sedimentation were ongoing at varying intensities, each cycle or episode affected the previous cycle. This evolution can be traced in the composition of the plutonic bodies, conglomerates, and sandstones. Thus, southern Alaska has evolved from young continental and oceanic crust to mature continental crust by magmatism, sedimentation, and accretion. Paleomagnetic data suggest that much of southern Alaska south of the Denali fault evolved as one or more terranes or plates in southerly latitudes from Permian(?) time that were accreted to North American continental terranes by early Cenozoic time. The temporal and spatial relation of these terranes throughout geologic time in relation to the tectonic cycles is intricate and complicated. The Alaska-Aleutian Range volcanoplutonic arc and the associated Queen Charlotte transition zone transform-fault system have dominated depositional and structural patterns in southern Alaska since the early Cenozoic. End_of_Article - Last_Page 953------------

Chronology of the structural and petrologic development of the southwest Sierra Nevada foothills, California: Summary

Metamorphic country rocks of the southwest Sierra Nevada foothills represent the southernmost exposures of the western foothills metamorphic belt. The rocks of the foothills belt in this region are disrupted and metamorphosed by the Cretaceous batholith to a much greater extent than to the north, probably owing to a deeper level of exposure. The oldest country rocks in the region consist of the Carboniferous to Triassic Kings—Kaweah ophiolite belt which served as basement for Jurassic volcanic arc and continent-derived clastic sequences (Saleeby, 1978, 1979). The history of ophiolite generation, sea-floor transport, continental margin accretion, and volcano-plutonic arc growth is recorded by numerous petrogenetic and deformational episodes. Useful time constraints have been derived for a significant number of these episodes by U/Pb and K/Ar geochronometry. These time constraints have facilitated the construction of an absolute chronology for the structural and petrologic evolution of the region. In this paper, the critical geological and geochronological relations are summarized. The isotopic data and their interpretation, and the detailed field relations of the rocks dated along with supporting petrographic and petrochemical data are presented in the complete article in Part II.

Evolution of the Late Cretaceous forearc basin, northern and central California

The Upper Cretaceous part of the Great Valley Sequence of California provides a unique opportunity to study deep-marine sedimentation, petrologic evolution, and tectonic evolution of a forearc basin. Actualistic models of submarine fan sedimentation and arc-trench evolution provide the basis for unraveling the complex depositional history of the bathyal to abyssal sediment deposited between the Sierra Nevada volcano-plutonic arc to the east and the Franciscan subduction complex to the west. Submarine fan components are lenticular stratigraphic units which can be correlated along strike on the basis of both paleontologic and petrologic data. The following depositional components are present: basin plain, outer fan, midfan, inner fan, slope, and shelf. Vertical successions of fan facies associations constitute retrograding and pro-grading suites that correspond, respectively, to onlapping and offlapping relations in the basin. Sedimentation rates are similar to those of other tectonically active flysch basins. Paleocurrents are predominantly southerly and westerly in the Sacramento Valley, and predominantly westerly in the San Joaquin Valley. Microfossil evidence and the lack of carbonate material suggest deposition below the Late Cretaceous calcite compensation depth. Dimensions and geometries of tectono-stratigraphic components of the Late Cretaceous arc-trench system are similar to those of modern arc-trench systems. The Late Cretaceous arc-trench gap widened by the prograde accretion of the Franciscan Assemblage (subduction complex) and the retrograde migration of the Sierra Nevada volcanic front (arc). Sediment dispersal systems expanded as the basin widened. The Java arc-trench system provides a modern analogue for the Late Cretaceous forearc basin, with sediment fed laterally from the arc and dispersed longitudinally along the basin axis.

SOUTHEAST ASIAN TIN GRANITOIDS OF CONTRASTING TECTONIC SETTING

The three major tin granitoid belts are, from west to east:a) Western: From Phuket to Tenasserim. The tin deposits are associated with highlevel Cretaceous adamellite, granite and pegmatite. The mineralization extends over a wide vertical extent.b) Main Range: From Bangka to South Thailand. The tin deposits are associated with deep-seated large-microcline granite and adamellite of late Carboniferous and late Triassic age. The mineralization is confined to the roof zones of the batholith.c) Eastern: From Billiton to Pahang-Trengganu. The tin-tungsten deposits are associated with adamellite to granite of Permian to mid Triassic high-level plutons. Mineralization extends over a wide vertical extent and there is an important Fe-Sn association.Only the Eastern Belt can be classified as Circum-Pacific type. It represents an epizonal volcano-plutonic arc characterized by plutonic rocks ranging from gabbro, through tonalite, granodiotite, adamellite to granite. Rhyolitic ignimbritic volcanic rocks are important.The Western Belt has some of the Eastern Belt characteristics, but lacks the range of plutonic rocks, and volcanic rocks are absent. However, the Tertiary opening of the Andaman Sea requires that the Burmese-Indonesian volcanic arc formerly was adjacent to the adamellite-granite belt.The Main Range Belt is interpreted as resulting from crustal anatexis of the leading edge of the western craton as it attempted to subduct beneath the Eastern volcano-plutonic arc following the late Triassic closure of the central marginal basin.

Geochronology of West Indonesia and its implication on plate tectonics

The results of radiometric dating of granitic rocks around Kotanopan near the west coast of Central Sumatra indicate an average age of 45 million years. Granites from the Lassi Mass in the Padang Highlands, Central Sumatra, and the Lampong Mass, South Sumatra, possess radiometric ages of ca. 112 and ca. 88 m.y., respectively. Granites and other rocks from the offshore areas north of Java indicate an average age of 100 m.y. Late Cretaceous granitic rocks are present in the islands of the Sunda Shelf namely Anambas (ca. 86 m.y.), Tembelan (ca. 85 m.y.) and Natuna (ca. 75 m.y.). Late Paleozoic granites possessing ages of ca. 276–298 m.y. are encountered in the basement rocks near Djambi, South Sumatra. The outcome of this radiometric age dating proves to be significant for it permits a fresh analysis of the geological evolution of Indonesia based on the plate-tectonics concept. The Tertiary volcano-plutonic arc exposed along the west coast of Sumatra can be traced to the south coast of Java. The corresponding subduction zone can be found in the islands west of Sumatra and the submarine ridge south of Java. The Late Cretaceous plutonic belt of Sumatra does not continue to Java but passes north of it, running however parallel to the subduction zone of Java. These two zones merge in the Meratus Mountains of Southeast Kalimantan. Sumatra was already a volcano-plutonic arc during Permian time, suggesting that since this Period the margins of at least four lithospheric plates have remained near the side of the active Sumatran arc. The presence of Permian volcanic and granitic rocks in the Malay Peninsula and West Kalimantan, and the results of the radiometric age determination of granitic rocks from the islands situated in the Sunda Shelf area, point to the existence of other Permian and Cretaceous volcano-plutonic arcs east and north of the arcs previously described in Sumatra and Java. Thus a double volcano-plutonic arc with opposing Benioff zones must have existed during Permian and Cretaceous time in this area. The Schwaner Mountains of West Kalimantan are considered to be the place where volcano-plutonic arcs of different ages have merged together. The correlative subduction zones have to be sought in the so-called Danau Formation of West Kalimantan and the northern part of the Kuching zone, the Sibu zone of Serawak situated north of the Schwaner Mountains. The evolution and complex geology of the western part of Indonesia can only be understood by the supposition of the existence of megaplates and sub-plates generated from spreading centers situated in the Indian Ocean and presumably in the area of the South China Sea, respectively.

Yakutat Group, an Upper Mesozoic Flysch and Melange Sequence in Southern Alaska: ABSTRACT

The Yakutat Group, part of a belt of highly deformed late Mesozoic rocks deposited in deep water and extending for more than 1,600 km along the Gulf of Alaska margin, underlies an area 200 km long by 15-30 km wide of the Saint Elias Mountains foothills and adjacent coastal plain. Bedded rocks of the Yakutat Group are dominantly Cretaceous graywacke and pelite with local pebble-cobble conglomerate and sparse pods of oolitic chert-nodule limestone. Interspersed with the bedded rocks are melanges composed of large blocks of competent rocks up to several kilometers in size engulfed in a pervasively sheared matrix of pelite or tuffaceous pelite. These clasts include externally derived or exotic greenstone, marble, metagraywacke, metachert, and diorite that are at least in part of Triassic(?) and Middle Jurassic age. Following deposition, the Yakutat was subjected to (1) pre-late Eocene compressive folding and thrusting, (2) regional zeolite to low greenschist-facies metamorphism with emplacement of early Tertiary granitic plutons, and (3) disruption by large-scale dextral shearing. Available data suggest that the Yakutat was deposited in an oceanic trench associated with a volcano-plutonic arc. Large exotic clasts were presumably incorporated in the melange by tectonic disruption and submarine sliding of older rocks exposed along the inner wall of the trench. A source for the sediments and exotic clasts does not exist in contiguous parts of the Saint Elias Mountains. The most probable provenance lies roughly 240 km southeast in the Chicago-Baranof-Admiralty Islands area of southeastern Alaska. Large-scale post-middle Eocene dextral slip on the Fairweather fault is postulated to explain separation of the Yakutat Group from its source area. End_of_Article - Last_Page 800------------

Trans-Andean Geophysical Profile, Southern Colombia

Negative Bouguer anomalies (−80 mgals) near the Pacific coast of southern Colombia define the position of the Tertiary Bolivar trough. Values increase eastward to a huge positive anomaly (+75 mgals) over Mesozoic “eugeosynclinal” rocks of the western Andes. This anomaly is part of the West Colombian gravity high, which extends from Panama into western Ecuador and is caused by shallow mafic crust. Bouguer anomalies are strongly negative (−220 mgals) over pre-Mesozoic(?) metamorphic rocks, Mesozoic(?) granitic bodies, and Tertiary to Holocene volcanic rocks of the central Andes between Pasto and Ipiales. The steep gravity gradient between the West Colombian gravity high and the negative anomaly of the central Andes represents the transition between mafic crust to the west and continental crust to the east. This zone parallels the Romeral-Cauca megashear system. East of the Andes, Bouguer anomalies range from −50 to −120 mgals over a Mesozoic-Tertiary basin of the Putumayo district, indicating that the crust there is thinner or denser than it is beneath the central Andes. Models derived from gravity data suggest that the crust is about 45 km thick under the south-central Colombian Andes. If this is correct, the crust must thicken southward along the strike of the Andes, as thicknesses of 70 km have been reported in the Andes of southern Peru, Bolivia, and northern Chile by Lomnitz (1962) and James (1971a). Such differing crustal thicknesses may reflect different intensities of tectonic activity, greater crustal thickness indicating more intense or rapid growth of the volcano-plutonic arc or foreshortening of an existing crustal section.

Detrital modes of new zealand graywackes

Graywacke, used here for any dark indurated sandstone, and argillite are the dominant pre-Tertiary rocks underlying much of New Zealand. The strata, mainly Permian to Jurassic, can be divided into two facies: (1) the western marginal facies of orderly, fossiliferous strata exposed in the New Zealand marginal syncline; and (2) the eastern axial facies of strongly folded and faulted, and partly schistose, rocks that occupy the keel of the New Zealand geosyncline. The two facies are in fault contact along the length of New Zealand. Each facies is 15–20 km thick, and records deposition, probably on oceanic crust, at a rate ≈125 ± 25 m/million years (m.y.) for ≈150 m.y. The western marginal facies apparently was deposited on a rapidly subsiding shelf that was positioned as a major topographic bench between an oceanic trench, site of sedimentation of the turbidites of the eastern axial facies, and a volcano-plutonic arc, probable provenance of the detritus in the two facies. The two facies were metamorphosed at low grade under relatively high pressures and low temperatures, probably in the sub-trench belt of paired metamorphic belts, with the provenance region being the sub-arc belt of relatively low pressure and high temperature. The graywackes of the western marginal facies apparently record a progressive erosional modification of the volcano-plutonic provenance, from a volcanic terrane that shed detritus into lower horizons of the sequence to a plutonic terrane that shed detritus into higher horizons of the sequence as dissection bit into the igneous source rocks of the arc. The most common graywackes of the eastern axial facies are more quartzose than those of the western marginal facies, and may have received plutonic detritus that bypassed the shelf between the arc and the trench. Feldspatholithic “volcanic” graywackes are poor in quartz, rich in pyriboles, bear mainly plagioclase feldspar, and contain mostly volcanic rock fragments. Lithofeldspathic “plutonic” graywackes are relatively high in quartz and mica, bear K-feldspar as well as plagioclase, and contain abundant metamorphic as well as volcanic rock fragments. Analogous paired suites of graywackes may be common in the circum-Pacific region.

Relations of andesites, granites, and derivative sandstones to arc‐Trench tectonics

Andesitic volcanogenic sequences, granitic batholith belts, and derivative graywacke‐arkose sedimentary successions are prominent rock assemblages associated with alpinotype peridotite‐gabbro belts and other characteristic tectonic features in orogenic regions or mobile belts where repeated crustal deformation and metamorphism have occurred. Field relations in the circum‐Pacific region indicate that andesitic eruptive suites and granitic intrusive suites are commonly consanguineous and roughly contemporaneous and that they have shed voluminous detritus into coeval graywacke‐arkose belts nearby. Modern systems of oceanic trenches and parallel magmatic arcs are probable analogues of the tectonic settings in which the three related rock assemblages formed.Data on crustal geophysics, trace‐element geochemistry, and strontium‐isotope ratios preclude participation of sialic crust in the generation of andesitic magmas at shallow levels but permit alternative hypotheses of primary partial melts from the mantle, derivative melts differentiated from primary basaltic melts, or melts from oceanic lithosphere slabs descending along inclined seismic zones beneath the volcanic arcs. In Quaternary andesitic suites, areal petrologic variations, particularly in potash content, are consistent tranverse to active volcanic chains regardless of longitudinal variations in crustal thickness. Levels of potash content in different suites correlate well with depths to the inclined seismic zone beneath, although significant scatter of points is apparent. Petrologic data from older andesitic terranes can be used to plot approximate positions and inclinations of paleoseismic zones.The anatectic hypothesis for the origin of magmatic plutons in intrusive batholiths is challenged by apparent comagmatic associations with andesitic eruptives, common sequences of intrusion from mafic to felsic, doubtful presence of suitable geosynclinal roots in some areas, available strontium‐isotope ratios, difficult geothermal inferences, and unexpected episodicity or periodicity of repeated intrusive events that are correlative throughout large longitudinal segments of batholith belts. Consistent positions of batholith belts along the trends of relatively high‐temperature and low‐pressure members of paired metamorphic belts suggest that the granitic plutons were emplaced in the roots of complex volcanoplutonic arcs, and that granitic intrusive magmas may be derived from the same deep sources as andesitic eruptive magmas. Transverse petrologic asymmetry within Mesozoic batholiths of western North America is reminiscent of the similar petrologic asymmetry within Cenozoic volcanic terranes, and may be used to construct speculative paleoseismic zones for the volcanoplutonic arcs whose roots the batholiths may represent.Graywacke and arkose sequences that lie on the Pacific side of andesitic volcanogenic and granitic batholith belts are composed mainly of first‐cycle volcanic and plutonic detritus and commonly form large parts of the relatively low‐temperature and high‐pressure members of paired metamorphic belts. Detritus eroded during and between successive episodes of volcanism and plutonism in the adjacent volcano‐plutonic provenances was deposited in parallel subsiding belts, where it was progressively buried as an inverse record of the successive magmatic increments to the arc regions. The graywacke‐arkose belts commonly include two parallel divisions. Distal facies of strongly deformed trench and continental‐rise deposits were ground against and beneath the seaward flanks of the volcanoplutonic arcs. Proximal facies of more orderly strata were deposited in sediment traps between trenches and arcs in the tectonic position occupied by shelves, slopes, and troughs of varied bathymetric character in modern arc‐trench systems.The interpretations in this paper attempt to bring petrologic inferences about orogenic rock assemblages in line with current mobilist tectonic concepts that are supplanting previous stabilist views. The formation of the three rock assemblages discussed is probably the principal means by which continental crust is formed from the mantle.