Triassic Volcanic Rocks Research Articles

The Triassic metabasalts of Dudin Krs, near Kosovska Mitrovica, Serbia

This paper presents the geochemical characteristics of the metabasalts of Dudin Krs, near Kosovska Mitrovica. The Dudin Krs is the easternmost occurrence of Middle Triassic rift-related volcanic rocks in the Dinarides. Generally, these rocks show similarities to other Triassic volcanic rocks of the Dinarides. They are high-magnesian, ol and ne-normative basalts, with low Zr/Nb and medium/low Ti/Zr and Ti/Y ratios. They exhibit transitional geochemical characteristics, between E-MORB and subalkaline basalts of continental rift zones. Their presence and geochemical affinity is evidence of rifting processes along the continental slope of the Adria block during Middle Triassic.

Triassic mid-ocean ridge basalts from the Argolis Peninsula (Greece): new constraints for the early oceanization phases of the Neo-Tethyan Pindos basin

Abstract The Middle Unit of the central-northern Argolis Peninsula, in NE Peloponnesus (Greece), is composed of several tectonic slices, locally including intact sequences of mafic volcanic rocks topped by radiolarian cherts. Although some of these sequences are Jurassic in age, many of them display a Triassic age based on biostratigraphical evidence. The petrological studies presented in this paper indicate that the Triassic volcanic rocks were generated in a mid-ocean ridge setting, and that they represent the oldest remnants of the Pindos oceanic crust so far recognized in the Subpelagonian zone. On the basis of immobile trace element analyses, two chemically distinct groups of Triassic lavas can be recognized in the various volcanic sequences. One group is represented by transitional-type mid-ocean ridge basalts (T-MORBs) displaying moderate light rare earth element (LREE) enrichment, and incompatible element abundances very similar to those observed in present-day T-MORBs. The other group exhibits a range of characteristics typical of many normal-type MORBs: that is, variable LREE depletion and flat N-MORB normalized patterns of incompatible element abundance. Moreover, many geochemical characteristics indicate that the various N-MORB type volcanic sequences originated from chemically distinct (heterogeneous) sub-oceanic mantle sources. Analogous to similar basalts from ophiolitic mélanges of the Dinaride-Hellenide belt, the T-MORBs from the Argolis Middle Unit are interpreted as having originated from a primitive mantle source variably enriched by an ocean-island basalt (OIB)-type component. In contrast, the contemporaneous occurrence of N-MORBs implies that, during the Mid-Late Triassic, oceanic spreading of the Pindos basin had already reached, at least in some sectors, a quasi-steady state involving only sub-oceanic mantle sources and their partial melt derivatives. Our model for the Triassic opening of the Pindos oceanic basin and its related tectonomagmatic evolution is largely supported by comparison with the Red Sea embryonic ocean, a modern analogous setting.

A Middle Jurassic Radiolarite-Clastic Succession from the Medvednica Mt. (NW Croatia)

On the NW part of Medvednica Mt. radiolarites with carbonate olistoliths, shales and siltites, matrix-supported conglomerates and basic volcanic rocks were investigated. This facies association is informally named the Poljanica unit. Major element geochemical data indicate deposition of radiolarites in the vicinity of the middle oceanic ridge, while sedimentological data indicate deposition in an area closer to the continent. Shales and siltites, as well as matrix-supported conglomerates, were deposited in short periods characterised by increased input of terrigenous material. Matrix-supported polymict conglomerates are composed of silicified shales, lithic graywackes, cherts and metabasalts, and were deposited by debris flow mechanisms as a consequence of synsedimentary tectonic activity. Carbonate olistoliths are composed of biomicrosparite, and jointly with deformed radiolarian cherts compose an olistostrome. Basic volcanic rocks represent high-Ti tholeiitic basalts formed in the MORB realm. Micropalaeontological investigation of radiolarite samples proved the Middle Jurassic (latest Bajocian - early Bathonian to late Bathonian - early Callovian) age of the Poljanica unit. Additionally, a new radiolarian species Theocapsomma medvednicensis n.sp. has been described. Conodont analyses from carbonate olistoliths in radiolarites proved their Triassic age. The investigated radiolarite-clastic succession is the result of subduction processes. Further continuation of this process caused incorporation of these deposits into the accretionary prism, where they were brought in direct contact with Triassic volcanic rocks and radiolarites (in the form of a tectonic melange). Based on the lithological similarities with the Middle Jurassic turbidite-olistostrome successions in the Western Carpathians and Northern Calcareous Alps, the study area is considered to be part of the Meliata-Hallstatt Ocean.

Upper Triassic Takla Group volcanic rocks, Stikine Terrane, north-central British Columbia: geochemistry, petrogenesis, and tectonic implications

Upper Triassic Takla Group volcanic rocks, Stikine Terrane, north-central British Columbia: geochemistry, petrogenesis, and tectonic implications

The nature of Triassic extension-related magmatism in Greece: evidence from Nd and Pb isotope geochemistry

The widespread Triassic volcanic rocks of Greece, dismembered during the Hellenide orogeny, are used to interpret the nature of Triassic rifting. Four assemblages of volcanic rocks are distinguished on geochemical criteria: (1) a predominant subalkaline basalt–andesite–dacite series with a high proportion of pyroclastic rocks; (2) minor shoshonites; (3) alkali basalt and (4) MORB. The stratigraphic and palaeogeographic distribution of these rock types is synthesized. New Pb and Nd isotopic data are used to discriminate between hypotheses suggesting that either subduction or extension was responsible for the Triassic volcanism. In the subalkaline basalt assemblage, εNd is negative with depleted mantle model ages >1.5 Ga. Pb isotopic compositions are mostly close to the very distinctive compositional field of Cenozoic extensional rocks of the Aegean area, with very high 207Pb/204Pb for relatively low 206Pb/204Pb ratios. These isotopic data confirm interpretations based on trace elements that subalkaline basalts were predominantly derived from melt-depleted peridotite in the sub-continental lithospheric mantle as a result of extension. Small areas of enriched hydrous mantle partially melted to yield shoshonitic magmas. Nd and Pb isotopic compositions of the alkali basalts are quite different from those in other rock types and suggest a HIMU mantle source component derived from a small plume, which also influenced MORB compositions. Distribution of these various rock types is used to constrain palaeogeographic reconstruction of Triassic micro-continental blocks.

The Cache Creek Terrane (British Columbia, Canada): The Remnant of a Permian Triassic Oceanic Plateau

The Canadian Cordillera is composed of lithospheric fragments, different in nature, age and origin. In British Colombia, the Stikinia and Quesnellia terranes represent two Permian-Triassic volcanic arcs which accreted to North America during the Late Jurassic. These two arc-terranes are divided by the Cache Creek terrane (CC) considered to be a far travelled oceanic terrane. The most common exposures of the CC in the Cache Creek type locality is a melange composed of blocks of N-MORB type pillow basalts, gabbros, serpentinites, upper Triassic radiolarian cherts and mudstones caught in a schistosed clayey matrix. In central British Colombia, near Fort St-James, CC consists of two assemblages: (1) thin massive basaltic flows and tufts interbedded with Permian pelagic limestones and (2) pillow basalts, diabases and tufts interbedded with Upper Triassic radiolarian cherts or graded bedded greywackes. The CC is divided from the Quesnellia arc-terrane by the Pinche strike-slip fault system. Tectonic slices are caught in the shear zone and are composed of cumulate gabbros intruded by basaltic dikelets, doleritic dykes, peridodites and Palaeozoic platform carbonates which are in fault contact with the CC Permian and Upper Triassic volcanic and sedimentary rocks. The Permian basalts are composed of olivine, clinopyroxene (cpx) and plagioclase microphenocrysts in a groundmass which includes plagioclase microlites, oxides and calcite-filled vesicules. They are geochemically similar to oceanic island tholeiites (OIT): (1) flat rare earth (BEE) patterns [(La/Yb)on = 2], slightly depleted in Light (L)REE relative to Heavy (H)REE, (2) Th-, Ta-, Nband Ti-enriched relative to Primitive Mantle (PM), (3) ~Nd(T=315 Ma) = +5.5 The Upper Triassic porphyritic basalts are composed of Ti-rich or not cpx, olivine altered in iddingsite and plagioclase replaced by sericite. They show alkaline to E-MORB affinities. The alkali basalts are LREE enriched relative to HREE [(La/ Yb)cn = 10 to 14] and their PM nomalized abundance spidergrams exhibit the typical humped pattern of OIB. The E-MORB tholeiites differ from the alkali basalts by slightly LREE depleted patterns and are less enriched in Nb, Ta, Zr and Hf. Moreover, they

Mesozoic Igneous Suites in Hungary: Implications for Genesis and Tectonic Setting in the Northwestern Part of Tethys

Mesozoic igneous rocks occur in various tectonic units of the Intra-Carpathian Area of Eastern Europe. These rocks were situated several hundred km apart from one another during their formation, and subsequent large lateral displacements resulted in their present positions. They formed during a relatively wide temporal range (Middle Triassic to Late Cretaceous) through different petrogenetic processes associated with the Mesozoic evolution of the northwestern part of Tethys. In the Transdanubian subunit of the Alcapa block, Middle Triassic calc-alkaline, intermediate-to-acidic, and potassic rocks occur as pyroclastics, lava flows, and dikes in the Bakony and Buda mountains. The Gemer-Bukk subunit of the Alcapa block comprises two different igneous series: (1) slightly metamorphosed Middle Triassic volcanic rocks of the Eastern Bukk Mountains, which can be divided into an older (Anisian-Early Ladinian) calc-alkaline, intermediate-to-acidic volcanic series and a younger (Late Ladinian) alkaline basaltic ser...

Structural evolution of the Alaska Juneau lode gold deposit, southeastern Alaska

The Alaska Juneau lode gold deposit is hosted by a series of polydeformed Permian to Late Triassic volcanic, pelitic, volcaniclastic, and mafic intrusive rocks. Rocks in the mine area have been sheared and metamorphosed to greenschist grade. Interpretation of rock fabrics indicates several generations of ductile and brittle deformation. Prior to mineralization, reverse shear occurred along northwest-striking and northeast-dipping ductile shear zones. Mineralization consists of Eocene auriferous quartz–carbonate veins, which cut the regional metamorphic fabrics. Mineralization was followed by reverse right-lateral shear along northwest-trending ductile–brittle shear zones. Two northwest-striking and steeply dipping vein sets host the bulk of the ore. Orientation of carbonate fibers within the quartz veins were used to determine the deformation regime that existed during mineralization. Plunge of the fibers indicate that down-to-the-northeast extension occurred synchronous with mineralization. Structural data support a model whereby the Alaska Juneau deposit formed after the peak of ductile deformation during a period of local extension. Localization of veins to areas of infolded phyllite and gabbro suggests that competency contrasts within host rocks enhanced vein emplacement. Veining may have been facilitated by a change from a contractional to a transpressive deformational regime which may have led to local extension and fluid migration to favorable deposition sites.

Stratigraphy and tectonic setting of the upper part of the Cadwallader terrane, southwestern British Columbia

The Upper Triassic to Middle Jurassic Cadwallader terrane lies on the northeastern edge of the Coast Plutonic Complex in southwestern British Columbia. Previous work on the Cadwallader Group, the basal unit of the terrane, suggested it was an Upper Triassic (Carnian to middle Norian) volcanic arc and related clastic rocks. Volcanism ceased in early Norian time. A detailed study of the upper part of the Cadwallader terrane (Tyaughton Group and overlying Last Creek formation) shows that it is a sedimentary sequence deposited on the fringe of the inactive Cadwallader magmatic arc. The Upper Triassic (middle to upper Norian) Tyaughton Group consists of nonmarine to shallow-marine clastic rocks and limestones that show sudden changes in depositional setting. The Lower to Middle Jurassic Last Creek formation, a transgressive sequence of clastic rocks, disconformably overlies the Tyaughton Group. The clastic rocks in the two units were derived from a mixed volcanic and plutonic source region that also included a minor metamorphic component and local lower Norian limestones. The stratigraphy of the upper part of the Cadwallader terrane records long-term thermal subsidence of the basin caused by cooling of the magmatic arc after volcanism ceased in the early Norian. The detailed stratigraphy of the upper Cadwallader terrane supports correlation of the Cadwallader with the Stikine terrane, along which it is currently structurally juxtaposed.

2 1/2 and 3 dimensional interpretation of magnetic anomalies in the central-southern Pyrenees (Spain)

Available aeromagnetic data have been used to interprete magnetic anomalies using 2 1/2 and 3 D techniques in an area that includes parts of the Axial Zone and of the South Pyrenean Thrust Sheets in the Spanish Pyrenees. The interpretation has been made with the help of field magnetic susceptibility measurements. The dip of the Ebro Basin basement to the north has been inferred for part of the magnetic profile. Large magnetically disturbing bodies with the top at about sea level can be interpreted as a stack of Paleozoic basement thrusts or, alternatively, as an accumulation of Triassic volcanic rocks. Some smaller anomalies are clearly associated with outcropping or buried basaltic rocks (ophites). The disposition of major magnetic anomalies suggests that shear movement may have affected part of the studied area.

Geologic framework, tectonic evolution, and displacement history of the Alexander Terrane

The Alexander terrane consists of upper Proterozoic(?)‐Cambrian through Middle(?) Jurassic rocks that underlie much of southeastern (SE) Alaska and parts of eastern Alaska, western British Columbia, and southwestern Yukon Territory. A variety of geologic, paleomagnetic, and paleontologic evidence indicates that these rocks have been displaced considerable distances from their sites of origin and were not accreted to western North America until Late Cretaceous‐early Tertiary time. Our geologic and U‐Pb geochronologic studies in southern SE Alaska and the work of others to the north indicate that the terrane evolved through three distinct tectonic phases. During the initial phase, from late Proterozoic(?)‐Cambrian through Early Devonian time, the terrane probably evolved along a convergent plate margin. Arc‐type(?) volcanism and plutonism occurred during late Proterozoic(?)‐Cambrian and Ordovician‐Early Silurian time, with orogenic events during the Middle Cambrian‐Early Ordovician (Wales orogeny) and the middle Silurian‐earliest Devonian (Klakas orogeny). The second phase is marked by Middle Devonian through Lower Permian strata which accumulated in tectonically stable marine environments. Devonian and Lower Permian volcanic rocks and upper Pennsylvanian‐Lower Permian syenitic to dioritic intrusive bodies occur locally but do not appear to represent major magmatic systems. The third phase is marked by Triassic volcanic and sedimentary rocks which are interpreted to have formed in a rift environment. Previous syntheses of the displacement history of the terrane emphasized apparent similarities with rocks in the Sierra‐Klamath region and suggested that the Alexander terrane evolved in proximity to the California continental margin during Paleozoic time. Our studies indicate, however, that the geologic record of the Alexander terrane is quite different from that in the Sierra‐Klamath region, and we conclude that the two regions were not closely associated during Paleozoic time. The available geologic, paleomagnetic, and paleontologic data are more consistent with a scenario involving (1) early Paleozoic origin and evolution of the Alexander terrane along the paleo‐Pacific margin of Gondwana, (2) rifting from this margin during Devonian time, (3) late Paleozoic migration across the paleo‐Pacific basin in low southerly paleolatitudes, (4) residence in proximity to the paleo‐Pacific margin of South America during latest Paleozoic(?)‐Triassic time, and (5) Late Permian(?)‐Triassic rifting followed by northward displacement along the eastern margin of the Pacific basin.

Northward displacement and accretion of Wrangellia: New paleomagnetic evidence from Alaska

Wrangellia is a vast terrane that accreted along the Pacific margin of North America from Oregon to central Alaska. Previous paleomagnetic studies of the Triassic volcanic rocks of Wrangellia in south‐central Alaska and British Columbia have given paleolatitudes of 10°–17°, which are anomalously low in comparison to results from similar‐aged rocks of nuclear North America. The discrepancy in paleolatitude implies substantial northward drift of the terrane relative to the craton. We have extended paleomagnetic sampling to the northern boundary of the terrane in the Mount Hayes and Healy quadrangles of the central Alaska Range. This collection of 46 Triassic lava flows yields a mean paleolatitude of 13.9° (α95 = 3.8°) the polarity of the data and hence the hemisphere of origin are not resolved with certainty. In contrast, Triassic paleomagnetic poles from the stable part of North America predict a paleolatitude of 42.1°N (α95 = 4.6°) for the region. The northernmost fragment of Wrangellia probably reached mainland Alaska in the middle Cretaceous, when Upper Jurassic and Lower Cretaceous flysch within the intervening basin underwent severe deformation. Tertiary transcurrent faulting has modified the positions of Wrangellian fragments in Alaska, but such displacements are probably minor, as indicated by paleomagnetic data from early Tertiary volcanic rocks that overlap the terrane.

Relation of Metallogenesis to Accreted Tectono-Stratigraphic Terranes in Alaska: ABSTRACT

Alaska consists of a collage of about 50 fault-bounded tectono-stratigraphic terranes of regional extent, as well as numerous smaller blocks. Each terrane possesses a characteristic stratigraphy and structure that differ markedly from those of neighboring terranes. Their grossly different stratigraphic and structural histories imply juxtaposition by large-scale transport from diverse sites of origin in various parts of the Pacific basin. The resultant mosaic of terranes records a long and complex history of accretion to the continental margin of North America. Parts of the terranes have been substantially modified by post-accretion faulting, intrusion and volcanism, and metamorphism, principally during the Cenozoic. These fundamental differences between terranes imply cor esponding differences in metallogenesis, because metallogenesis is directly related to the geologic history of the rocks hosting mineral deposits. Consequently, a metallogenic model can be constructed that predicts: (1) differences in mineral deposits that formed during the origin of various dissimilar terranes; (2) differences in mineral deposits that formed during the transport and accretion of various dissimilar terranes; and (3) similarities in mineral deposits that formed within adjacent terranes after accretion. Three studies illustrate this model relating markedly different syngenetic mineral deposits, in three dissimilar terranes, to the particular origin of each terrane. The three terranes and their syngenetic mineral deposits are: (1) the Mississippian shale, chert, and tuff of the Kagvik terrane of the northwestern Brooks Range, in Arctic Alaska, which hosts extensive stratiform Zn-Pb-Ag-Ba sulfide deposits; (2) the late Paleozoic island-arc volcanic rocks of the Wrangellia terrane, in southern Alaska, which hosts volcanogenic Cu-Ag sulfide deposits; and (3) the Triassic silicic volcanic rocks of the Alexander terrane in southeastern Alaska, which hosts volcanogenic Zn-Pb-Ag-Ba sulfide deposits. End_of_Article - Last_Page 979------------

Petrology of the Turnagain ultramafic complex, northwestern British Columbia

The Turnagain ultramafic complex is an Alaskan-type complex of possible Late Triassic age. It has a central zone of dunite and an outer zone of wehrlite, olivine clinopyroxenite, clinopyroxenite, dunite, hornblendite (rare), and basic hornblende and plagioclase-bearing rock (rare). The complex is largely fault-bounded. The central dunite intrudes the pyroxene-bearing rocks in one area; elsewhere the two zones are gradational. Small-scale layering occurs locally, and is most common in the outer zone. Layering dips steeply due to folding during regional deformation. The ultramafic rocks are generally only partly serpentinized, and consist largely of olivine, clinopyroxene, pargasite, phlogopite, and chrome spinel. The outer zone contains sporadic concentrations of iron–nickel–copper sulfides. Olivine is most magnesian (Fo94.9) in the central dunite, and most iron-rich (Fo80.2) in olivine clinopyroxenite. Clinopyroxene is diopside and follows an iron-enrichment trend. Minor-element contents of minerals indicate that crystallization caused magmatic impoverishment in Ni and Cr, and enrichment in Ti and Al. The primitive Turnagain magma was probably ultrabasic, extremely magnesian, and poor in Al and Ti. Differentiation followed an alkalic trend, possibly at relatively low oxygen fugacity compared to other Alaskan-type complexes. The complex may have formed in a subvolcanic magma chamber, and thus be related to nearby Upper Triassic volcanic rocks.

On the Origin of Ultramafic Rocks

It is argued that most alpine ultramafic bodies originated as cumulates in basic magma chambers high in the crust. From some of these chambers, magmas were intruded upward or extruded, leaving sill-like ultramafites behind. These were subsequently folded or dismembered by faulting, and, because of their high density, subsided during tectonism, to form cold, fault-enclosed intrusions. In other places, tectonism interrupted crystallization of the magmas, magma chambers were deformed, loose cumulates and hot coherent cumulate rocks slumped and slid, and hot masses, as crystal mushes and hot lenses, worked down fault zones. These hot masses were subject to high temperature recrystallization and metasomatism, as water from adjacent rocks permeated them, forming veins of pyroxenite and dunite, and intercumulus plagioclase was destroyed. Contact aureoles were formed, but some were abandoned as the cooling ultramafites descended. The Bay of Islands complex and the Great Dyke are considered to be examples of bodies whose origins as stratiform differentiates is clear but which show some of the features of alpine ultramafic masses. It is argued that Franciscan peridotites are differentiates that have been folded or have descended along faults during tectonism. It is hypothesized that ultramafites of British Columbia are complementary to late Paleozoic or Triassic volcanic rocks. Zoned ultramafic complexes of Alaska, British Columbia, and the Urals, currently believed by many to have formed by crystallization of ultramafic magmas, are re-examined. It is concluded that the central peridotites of these consist of cumulates from basic magmas and have been dropped as crude cylinders along ring fractures or are downward intrusions of ultramafic crystal mushes. Their zoning is due to subsequent metasomatism by hydrous fluids that were guided by marginal fractures. Pipes of the Bushveld Complex may have formed entirely by replacement along and adjacent to “gas chimneys.”

Paleomagnetic Differentiation and Correlation of the Late Triassic Volcanic Rocks in the Central Appalachians (with Special Reference to the Connecticut Valley)

During the late Triassic and early Jurassic, the location of the earth9s magnetic pole changed rapidly. As a result of these changes the inclination of the magnetic field in North America increased considerably in a relatively short time. The igneous rocks emplaced during the late Triassic and early Jurassic, show thermo-remanent directions (T.R.M.) of magnetization with progressively higher positive inclinations. In Connecticut four periods of late Triassic-early Jurassic volcanic activity have been dis­tinguished geologically. They are from old to young: the Talcott, Holyoke, Hampden, and Higganum volcanic events. The mean inclination of the T.R.M. is + 12° for the Talcott, + 25° for the Holyoke, + 42° for the Hampden, and + 35° for the Higganum volcanic units. This paleomagnetic stratigraphy enables one to differentiate between the volcanic units and to correlate the units of one basin, with contemporaneous outflows or intrusions in other basins. By means of their distinctive paleomagnetic data it is possible to correlate the volcanics of the Newark and Gettysburg basins (Pennsylvania) with the Holyoke volcanic event, and the vol­canics of the Bay of Fundy (Nova Scotia) with the Hampden volcanic event of Connecticut. Such correlations suggest the possibility of a northeastward displacement of the volcanic activity in late Triassic time. This would indicate a progressive northeastward expansion of the broad geanticlinal arching of the Appalachians in the early Mesozoic time. The origin of the rapid shift is unknown. It appears, however, that the shift is related to a high order reversal of the magnetic field. The shift occurred in an interval of relatively low field strength separating the Permian positive magnetic era from the Cretaceous negative magnetic era. The same interval was characterized by an increase in the frequency of lower order reversals (magnetic epochs and events).

Les roches volcaniques du Trias inferieur du versant nord des Pyrenees

Abstract Detailed petrographic and chemical analyses of the principal zones of occurrence show that volcanic rocks interbedded in detrital deposits at the top of the Permo-Triassic section of the north flank of the Pyrenees (France), or occurring in the boundary between the Permo-Triassic and Muschelkalk (Triassic) limestones, are not melaphyres or basalts, but are albitophyres, orthoalbitophyres, and potassic spilites resembling Triassic volcanic rocks of the outer Alps.

Petrogenetic Significance of Geosynclinal Andesitic Volcanism Along the Pacific Margin of North America

The prevalent concept of magmatic evolution in orogenic belts holds that ophiolitic eruptions are characteristic of the geosynclinal phase and that felsic magmas first appear on a large scale during the orogenic phase of emplacement of granitic batholiths. In the Fraser Belt of western North America, however, the geosynclinal Jurassic volcanic rocks are dominantly fragmental augite andesites; subordinate rock types include dacite and rhyolite, as well as basalt. Typical Jurassic andesitic rocks are chemically similar to continental Tertiary andesites of the same region; the more mafic Jurassic rocks are chemically similar to high-alumina basalts. The geosynclinal Permian and Triassic volcanic rocks of the Fraser Belt are also dominantly andesitic and closely resemble the Jurassic rocks. The chemical similarity of the Late Mesozoic batholithic rocks to the geosynclinal andesites suggests that the granitic magmas possibly formed in part by partial fusion of geosynclinal volcanic rocks. The andesitic character of the geosynclinal volcanic rocks indicates that mobilized sial, either as a contaminant of mantle-derived basaltic magma or as primary andesitic magma, contributed substance to the geosynclinal volcanism. The evidence for a subjacent sialic crust during the geosynclinal phase and chemical similarities among the geosynclinal volcanic rocks, the orogenic granitic plutons, and certain postorogenic volcanic rocks suggest that the fundamental character of the crust beneath the Fraser Belt has not changed since mid-Paleozoic time.