Modern Volcanic Arcs Research Articles

A new stratigraphy for the Tertiary volcanic rocks of the Nicaraguan Highland

Three major volcanic events have occurred in Nicaragua since middle Tertiary time. First, the Oligocene was dominated by rhyolite shield volcanism, producing the Highland ignimbrite. Second, creation of northeast- and later northwest-striking dilatational fracture zones allowed the extrusion of basaltic to andesitic magmas along the Pacific coast, constructing the northwest-oriented Coyol volcanic arc. This Miocene arc can be followed all along the Pacific coast of the Chortis block. Third, a Pliocene-Pleistocene southwestward shift of arc volcanism created the modern volcanic arc. This volcanic history is supported by a new stratigraphy based on volcanic centers and related facies. The old cyclic, layer-cake stratigraphy of the Coyol Group has been abandoned.

Geology of the Hazelton Volcanic Belt in British Columbia: Implications for the Early to Middle Jurassic evolution of Stikinia

The Hazelton Group is an Early to Middle Jurassic volcanic and sedimentary succession that was deposited on the Stikine Terrane (Stikinia) of the Canadian Cordillera. The group is distributed across the entire width of Stikinia, a distance of about 300 km. Prior to post‐Hazelton contractional deformation, the width across which volcanism had occurred was at least 450 km. A review of chronologic and stratigraphic information corroborates an earlier suggestion that the Hazelton Group formed as a pair of coeval, partly subaerial volcanic chains separated by a subsiding, mainly sedimentary marine basin (Hazelton Trough). The trough developed in response to extension of Stikinia during the latest Triassic and/or earliest Jurassic, and remained the locus of moderate extension during deposition of the Hazelton Group. Volcanic rocks of the group are diverse, comprising subaerially and subaqueously deposited lavas and volcaniclastic rocks of mafic to felsic composition. Calc‐alkaline to tholeiitic characteristics, and depletion of high‐field‐strength elements relative to alkali and alkaline earth elements indicate a subduction‐related origin. The degree of alkalinity (medium‐ to high‐K) is fairly consistent throughout the group. The original width across which volcanism occurred is two to five times greater than widths of modern volcanic arcs. The anomalous width cannot be accounted for by time transgression of a single volcanic arc, because the two volcanic chains were coeval. Extremely low‐angle subduction of a single oceanic plate is also rejected as an explanation because alkalinity does not differ significantly between the two volcanic chains. Genesis of the Hazelton Group above a single subduction zone is therefore considered unlikely. We propose a model in which a pair of oceanic plates were subducted beneath Stikinia from opposite sides, in a manner analagous to the tectonic regime of the Philippine archipelago. Subduction angles of the two downgoing plates are postulated to be 30° to a depth of 150 km, a configuration consistent with a postulated forearc width of about 175 km on each side. Stikinia is thereby considered to have been an 800‐km‐wide microplate on which two island arcs developed. The Hazelton Trough was a back‐arc region common to both volcanic arcs. During deposition of the Hazelton Group, Stikinia was not part of the Intermontane Superterrane, an amalgam composed of more inboard terranes. Rather, it belonged to an oceanic microplate that accreted to the inboard terranes in late Early Jurassic to early Middle Jurassic time, during which Hazelton volcanism waned and ended.

The Kan River Gneiss terrane of central Côte D'Ivoire: mylonitic remnants of an ancient magmatic arc?

The Kan River Gneisses (KRG) which crop out to the east of the Birrimian Fetekro supracrustal belt in central Cote d'Ivoire are described. They comprise a TTG intrusive suite, part of which has been metamorphosed to amphibolite facies and extensively deformed by strike-slip deformation. These gneisses are considered to have been originally linked to the better known Dabakala Gneiss to the north. The eastern and western boundaries of the KRG are defined by major crustal shear zones which separate discrete lithostratigraphic terranes. To the east, the Comoe flysch basin lies beyond the Dimbokro strike-slip shear zone. The Boni Andokro shear zone marks the boundary between the KRG and the Toumodi Volcanic Group (TVG) to the west, where there is evidence of shortening and transcurrent movement. The KRG are geochemically similar to modern volcanic arc granites. They are clearly petrologically distinct from the calc-alkaline intrusive granitoids which intrude the TVG, and which geochemically resemble modern volcanic arc and syn-collisional granites. The KRG terrane resembles a deformed magmatic arc. In view of the nature of its eastern and western boundaries, its relations with neighbouring terranes are considered suspect.

Late Proterozoic tectonic model for the Avalon Terrane in Maritime Canada

The Avalon Terrane in Maritime Canada extends from Cape Breton Island through northern Nova Scotia (Antigonish and Cobequid highlands) to southern New Brunswick. The terrane contains abundant late Proterozoic volcanic rocks (∼570–680 m.y. old). The geochemistry of the volcanic rocks of southern Cape Breton Island and southern New Brunswick indicates that they were erupted in a volcanic arc environment. In both regions the mafic rocks display a remarkably similar compositional zonation which resembles the across‐arc variations observed in modern volcanic arcs. The progressive compositional changes that include a transition from island arc tholeiites along the southeastern coasts to calc‐alkaline rocks inland to the northwest are probably due to a northwest dipping subduction zone with a trench located to the southeast. In the Antigonish and Cobequid highlands, continental tholeiites and calc‐alkaline volcanic rocks are interbedded and overlain by turbidites that have probably been deposited in a back arc or intra‐arc environment. The superimposed positive gravity and magnetic anomalies that may be traced discontinuously from the Grand Banks through Nova Scotia (Collector Anomaly) and along the northern side of the Gulf of Maine to near Boston, probably represent late Proterozoic ophiolitic remnants preserved by incomplete suturing along a late Proterozoic plate boundary. Northeast trending segments are inferred to mark subduction zones, and the E‐W portions could correspond to transform faults. Thus the present geographical distribution of the Late Proterozoic volcanic rocks is explained by a tectonic model involving a single subduction zone offset by two late Proterozoic transform faults located approximately along the Minas Fault and the Scatarie Ridge.

Arc rifting of the Carolina terrane in northwestern South Carolina

Recent mapping and whole-rock geochemistry studies demonstrate that mafic metavolcanic rocks found along the boundary between the exotic Carolina terrane and the Inner Piedmont formed in a subduction-related volcanic are and do not represent the Iapetan suture. Mafic metavolcanic rocks are spatially and genetically related to zoned mafic-ultramafic intrusive complexes. These rocks are similar to those found in other ancient and modern volcanic island arcs where ankaramites and picrites are well known, and they are locally associated with zoned complexes, e.g., Sierra Foothills-Klamath Mountains of the western U.S. Cordillera. We propose that prior to accretion to Laurentia in the early to middle Paleozoic, the Carolina arc terrane underwent an episode of intea-arc rifting which allowed primitive arc magmas to ascend and erupt without significant crystal fractionation or lithospheric assimilation. This interpretation may help resolve some stratigraphie problems in the eastern part of the Carolina terrane (Carolina slate belt).

The Archaen volcanic facies in the Migori segment, Nyanza greenstone belt, Kenya: stratigraphy, geochemistry and mineralisation

The Migori segment is an 80 by 20 km portion of the Nyanza greenstone belt which forms the northern part of the Archean Tanzanian Craton in western Kenya, northern Tanzania and southeastern Uganda. It consists of two volcanic centres, each with central, proximal and distal volcanic facies, comprising the Migori Group, the Macalder and Lolgorien Subgroups, and eleven volcano-sedimentary formations. The centres are separated by a basin of tuffs and greywacke turbidites. The volcanics are bimodal mafic basalt and dolerite ( Zr/Y = 3.8 − 6.5, La N/Yb N = 1.0 − 2.4) , and felsic calc-alkaline dacite-rhyolite ( Zr/Y = 10 − 21, La N/Yb N = 19 − 42 ) and high-K dacite ( Zr/Y = 9 − 16, La N/Yb N = 21 − 22 ). Felsic units form approximately three-fourths of the volcanic stratigraphy. Basalts, calc-alkaline dacites and rhyolites were deposited in a submarine environment, but the voluminous high-K dacites were erupted subaerially. The turbidites contain units of iron-formations. Granitic intrusions are chemically continuous with the high-K dacites. The felsic volcanics are anologous to those found at modern volcanic arc subduction settings involving continental crust. The Macalder ZnCuAuAg volcanogenic massive sulphide deposits is in central facies basalts-greywacke-rhyolite. Gold mineralisation occurs in proximal facies tuffs and iron formation, and in oblique and semi-conformable quartz veins. Greenstones in the Nyanza belt are dominated by calc-alkaline felsic volcanics in constrast to the komatiite-tholeiitic basalt volcanism in the Kaapvaal Craton of South Africa, and a mixture of the two types in the Zimbabwe Craton.

Seismic velocities from the Kohistan Volcanic Arc, northern Pakistan

The rocks bounded by the Main Mantle Thrust and the Northern Suture in northern Pakistan constitute an exhumed section through a Cretaceous volcanic arc. Samples have been collected from all the principal lithological groups of the arc, and P and S-wave seismic velocities have been measured in the laboratory with the prime objective of comparing the velocities with those determined by seismic refraction experiments on modern volcanic arcs. Velocities were measured at up to 0.7 GPa and consideration has been given to the effect of confining pressure, pore pressure and temperature and to the deformation and metamorphism involved in the Himalayan collision. The reconstructed velocity section through the arc shows a distinct ‘upper crust’ comprising granitic-dioritic intrusions, metasediments and volcanics with a P-wave velocity of 6.2–6.4 km S −1 depending on the parameters used. Beneath, the ‘lower crust’ of amphibolitcs and pyroxene granulite has a velocity mainly about 6.4–6.7 kms −1 , though the garnet granulitcs extend to 7.8 km S −1 . The more mature arc with a higher proportion of granitic rocks would show a slightly lower upper crustal velocity than the younger arc. Velocity inversions might be expected with a thermal gradient of as little as 10–15 ° C km −1 , depending on the pore pressure. In general, the proposed velocity structure is comparable with that of volcanic arcs and with many other sections of the continental crust.

Seismic stratigraphy of the central Tonga Ridge

Seismic stratigraphic analyses of single- and multichannel seismic reflection profiles from the central part of the Tonga arc system, SW Pacific, are used to interpret its geological evolution. On the central Tonga Ridge (a high-standing part of the frontal arc, as defined by Karig (1970)) and its flanks between 18°S and 22°S, we identify and map six seismic sequences overlying inferred lower(?)—middle Eocene volcanic (acoustic) basement. Available island sections and boreholes, along with seismic facies, suggest that the upper Eocene to upper Miocene stratigraphic succession on the central Tonga Ridge consists of volcaniclastic turbidites and hemipelagic sediments. In contrast, carbonate deposition predominates from the early Pliocene to the present. We interpret major hiatuses during the early Oligocene, early Miocene and early Pliocene. Based upon the available seismic data and previous results, we infer that the extreme eastern Tofua Trough, a partially filled basin that now separates the central Tonga Ridge from the Tofua Ridge (the modern volcanic arc), may be the site of latest Eocene or Oligocene extension which may be related to rifting and back-arc spreading in the South Fiji Basin to the west. Another late Miocene or early Pliocene phase of rifting deformed strata deposited in the Palaeogene basin, and led to creation of at least part of the modern Tofua Trough. Available evidence suggests that the Neogene extension is related to initiation of sea floor spreading in the Lau Basin. Widespread carbonate deposition beginning in the early Pliocene suggests a decrease of volcanic activity in the vicinity of the central Tonga Ridge. An apparent N-S migration of Miocene(?) and younger depocentres may be related to sequential uplift of parts of the central Tonga Ridge in response to progressive southward subduction of the Louisville Ridge at the Tonga Trench.

Intra-arc sedimentation and tectonism: Late Paleozoic evolution of the eastern Klamath terrane, California

Rocks of the eastern Klamath terrane (Redding section) in northern California record deposition, magmatism, and tectonism in an island-arc setting. Volcanism and sedimentation persisted for more than 220 m.y., spanning Devonian to Middle Jurassic time, and yielding thick successions of water-laid volcanic debris and lavas, with interstratified carbonate and clastic sequences. This study focuses on the upper Paleozoic rocks within the section. The oldest rocks studied, within the Upper Devonian(?) and Mississippian Brag-don Formation, contain epiclastic debris that was deposited in a submarine-fan setting. Sandstone compositions indicate that quartzose sedimentary and metasedimentary rocks formed the source area for epiclastic debris and that volcanism persisted throughout sedimentation. Transitional shallow-water facies in the upper part of the Bragdon interfinger with the overlying Baird Formation, which contains abundant volcanic debris. Volcanogenic shelf and strandline deposits characterize the lower part of the formation, and volcaniclastic turbidites characterize the upper part. The Baird Formation is locally interfingered with the overlying Lower Permian McCloud Limestone, which records two stages of Early Permian carbonate platform development: early Wolfcampian and late Wolfcampian to early Leonardian. Only the younger of the two platforms is present within the study area, and it contains more distal facies than those preserved to the south. The strata contain abundant fusulinids and corals which provide age constraints for carbonate-platform development and form the basis for regional biogeographic correlation. The time-transgressive Upper Permian Bollibokka Group (Nosoni and Dekkas Formations) succeeds the McCloud Limestone, comprising thick sequences of volcanogenic debris, intercalated lava, and, in the upper part, lenses of shallow-water limestone. The Upper Permian strata reflect volcanism and rapid subsidence within the are. All upper Paleozoic units were intruded by small gabbroic bodies and diabase dikes which may be partly cogenetic with Upper Permian basalts in the upper Dekkas Formation. The upper Paleozoic succession records local pulses of magmatism and variable subsidence rates within the eastern Klamath arc. Mississippian epiclastic rocks reflect uplift of the volcanic-arc basement, which contained rocks similar to those now exposed in the adjacent Yreka terrane or Shoo Fly Complex. Pulses in volcanism were restricted to mid-Carboniferous and Late Permian time. Diminished volcanism during the Early Permian allowed development of carbonate platforms. Rapid fluctuations of sedimentation and sub-sidence rates; pronounced lateral facies and age variations; accumulation of thick, volcanic-debris aprons during episodic volcanism; and sporadic persistence of volcanism during apparent lulls characterized deposition within this succession and are common processes in modern volcanic arcs. The stratigraphic record is composed predominantly of volcanogenic rocks and lesser carbonates, reflecting events and processes within the arc itself. Evidence for nearby basins or landmasses is sparse. The Mississippian epiclastic rocks, however, which were derived from the arc basement, reflect an ultimate sialic crustal source. Comparison of volcanic-arc evolution with the late Paleozoic evolution of continental margin and basinal rocks to the east indicates possible paleogeo-graphic ties between the eastern Klamath volcanic arc and rocks of the Golconda allochthon or the North American continent.

Deep-water facies and depositional setting of the lower Conception Group (Hadrynian), southern Avalon Peninsula, Newfoundland

The lower part of the Hadrynian Conception Group is exposed in coastal outcrops near Holyrood Pond, St. Mary's Bay. Deep-water sediments of the Mall Bay Formation (>800 m) are overlain by glaciogenic debris flows of the Gaskiers Formation (250–300 m) and then by renewed turbidite deposition in the lower part of the Drook Formation (1000 m examined; formation thickness 1500 m). The main facies in the Mall Bay and Drook formations are thinly to very thickly bedded sand stone–mudstone couplets (turbidites), amalgamated in the thicker and coarser beds, and graded-stratified siltstones–mudstones. These are interpreted as deposits on an extensive basin floor and on the lower and middle parts of small-scale submarine fans that prograded from fringing volcanic islands. Paleocurrent data indicate a complex paleogeography dominated by active volcanic islands, probably like some modern volcanic arcs. The Gaskiers Formation was generated by glaciation of these islands.The Malll Bay Formation contains lenticular and wavy bedded siltstones–mudstones interpreted as the deposits of bottom currents, perhaps flowing parallel to the contours of the volcanic islands. In Phanerozoic sediments, bottom-current deposits in this setting would probably be thoroughly bioturbated.

The Middle Triassic magmatic-tectonic arc development in the Southern Alps

The geodynamic meaning of the Middle Triassic magmatic and tectonic events in the Southern Alps is still uncertain. The main problem is the significance of the compressional tectonic structures recognized in the central Dolomites. In fact these structures are incompatible with the extensional geodynamic interpretation (crustal rifting) assumed by several authors. Therefore the compressional tectonic elements of the Dolomites have been mostly ignored or not seriously taken into account, or considered as local phenomena due to the volcanic-tectonic interplay. Other more modern interpretations are founded on regional transcurrent and/or transpressive tectonics. Recent discovery of a Middle Triassic thrust system located in the Piave River area (some tens of kilometres east of the central Dolomites) lends more credence to the compressional Middle Triassic event. The magmatic series of volcanic and plutonic Middle Triassic rocks display an “orogenic” character, with typical calc-alkaline trends recognized across the whole Southern Alps as well as in the Dinaric and Hellenic domains. Such a character is rather uncommon in the crustal evolution by rifting but is more in accord with a compressional geodynamic setting. Furthermore the time-space distribution of the South Alpine magmatic rocks, coupled with their petrochemical affinity, are close to the modern volcanic arcs. Nevertheless the general geodynamic context of the Southern Alps, owing to their pronounced ensialic character, does not fit the oceanic model.

Crustal structure of the outer Banda arc: New free-air gravity evidence

Marine gravity data from the Banda arc in the vicinity of western Timor and the central Savu Sea, integrated with geophysical and geological information, support the postulated subduction of Australian continental crust beneath the Timor trough and the island of Timor. Gravity analysis also indicates the presence of a mass deficit in the upper mantle or crust beneath the entire Savu Sea. This mass deficit is interpreted to be related to a combination of 1. (1) subducted, lighter continental crust, 2. (2) downward flexing of the upper crustal plate in the forearc area, and 3. (3) possibly anomalous, low-density upper mantle below the Savu Sea. The North Sumba ridge, a bathymetric and structural high dividing the Savu Sea, is underlain by high-density material possibly related to an older volcanic arc located between the present-day volcanic arc and Timor. The crust beneath the northern Savu basin appears nearly oceanic in character but thickens beneath the modern volcanic arc. Finally, our analysis suggests thickening of crust beneath Timor forming a root in relation to the Savu Sea crust to the north.

Petrology and geochemistry of lower to middle Ordovician igneous rocks in Wales: a volcanic arc to marginal basin transition

Volcanic activity was widespread in Wales during Ordovician times. There are marked petrological and geochemical differences between late Tremadoc volcanism and that of Arenig to Llandeilo age. The earlier activity, represented by the Rhobell Volcanic Complex and the Trefgarn Volcanic Group, comprises a fractionation continuum which is basalt-andesite-dacite-rhyodacite and is similar to that of modern volcanic arcs. In contrast, the later activity, as seen in the Fishguard Volcanic Complex and the Aran Volcanic Group, is represented by basalt-rhyolite bimodal sequences. The basic rocks are similar to those of ensialic marginal basins and the acidic magmas were most probably generated by crustal fusion. The contemporaneous Builth Volcanic Group has some arc-like characteristics but the variation is here interpreted as due to the influence of subduction zone components in marginal basin magma generation. The change from the Tremadoc to later volcanism reflects an arc to marginal basin transition.

The influence of crustal structure on compositions of subduction-related magmas

A survey of Sr isotopic ratios and other compositional features of subduction-related magma suites reveals significant correlations between these averaged parameters and characteristics of the underlying crust (i.e., thickness, composition, and age). These observations lead to the conclusion that crust and(or) mantle rocks in the hanging walls of subduction zones are involved in modification of primary mafic magmas (typically basalt or basaltic andesite). It is proposed that mafic magmas will stagnate within the crust or uppermost mantle where they may differentiate and react with wall rocks. The extent to which such processes manifest themselves will depend upon details of the local crustal structure. In particular, the composition and age of the crust will strongly influence such parameters as Sr, Nd and Pb isotopic compositions. Such data strongly indicate the involvement of crustal rocks in locales underlain by old sialic crust (e.g., central Andes). Depending upon the level of magma stagnation and evolution within the crust, different trends in isotopic composition may result. These isotopic trends may be enhanced by partial melting of the wall rocks to produce relatively silicic anatectic magmas, and locally they may reflect subduction of continental sediments. Interpretation of the isotopic data may be more ambiguous in locales underlain by younger and more mafic continental crust (Cascades, E Eleutians) and those underlain by oceanic crust owing to the similarity in isotopic composition of primary magmas and the latter crustal materials. Yet some degree of crustal involvement in magmatic evolution seems highly probable even in these more primitive terranes. Consequently, most island arc magmas, and especially those more evolved than basalt, are probably not primary in the sense that they do not represent direct melts of the upper mantle. Studies of arc volcanic rocks may yield misleading conclusions concerning processes of magma generation related to subduction unless evolutionary processes are defined and their effects considered. It appears that modern volcanic arcs provide a poor analog for models of early crustal development because the modern mantle-derived magmatic components are more mafic in composition than average continental crust.

Systematic variation in chemistry and Nd-Sr isotopes across a Caledonian calc-alkaline volcanic arc: implications for source materials

Sr and Nd isotope compositions, and major and trace element chemistry, are reported for 42 primitive calc-alkaline lavas from the Siluro-Devonian Scottish Old Red Sandstone. Northwestward increases in concentrations of Sr, Ba, K, P and LREE associated with northwest subduction of Iapetus ocean crust [1] are not accompanied by change in Rb, Th or Ti-group elements. These relationships show that the spatial variation cannot be simply the result of change in the degree of partial melting with depth. Initial 143Nd/ 144Nd expressed as ε Nd at 410 Ma shows a wide range from +6.4 to −3.7, and in contrast to ε Sr decreases markedly to the northwest. Samples from the Midland Valley show a strong anticorrelation between ε Nd and ε Sr , with the correlation band offset to more radiogenic compositions than that defined by modern oceanic volcanic rocks (the “mantle array”). Such an offset is common in modern volcanic arc magmas, but cannot be explained by addition to the overlying mantle of a single component enriched in radiogenic Sr derived from subducted lithosphere. Southwest Highland lavas lie in a completely separate field on an ε Sr-ε Nd diagram: most fall within the mantle array, but samples with highest concentrations of Sr, etc., are offset to less radiogenic compositions. Consideration of the chemical and isotopic relationships with respect to several models of crustal contamination shows that the additional LREE in the northwest cannot be derived from a crustal source. The data imply the existence beneath Scotland of a large region of mantle with time-integrated LREE enrichment, parts of which may have anomalous Nd-Sr isotopic compositions less radiogenic than the mantle array. Midland Valley lavas are probably derived from a long-term LREE-depleted mantle source modified by interaction with subducted oceanic lithosphere. The coexistence of two mantle sources with differing histories is best explained by vertical stratification, with deeper, more LREE-enriched layers accessible only to magmas formed at great depth in a subduction zone. The spatial variation probably in part reflects this mantle layering and in part the operation of some mantle zone-refining process that permits efficient concentration of incompatible elements from large regions of mantle.

Geochemical evidence for paired arcs in the Permian volcanics of southern New Zealand

Although remnants of a Permian volcanic arc in the South Island of New Zealand have hitherto been considered to comprise a single volcanostratigraphic unit, recent work in the southern part of the arc has revealed the presence of two lithologically distinct terranes, only one of which is definitely Permian in age. Major and trace element analyses of representative specimens show that these terranes are geochemically distinct. Comparison with modern volcanic arcs indicates that this ancient arc in the South Island was a paired arc system made up of primitive island arc tholeiites to the east and more evolved andesitic rocks to the west.

Timiskaming volcanism in the Kirkland Lake area, Ontario, Canada

The petrography and chemistry of the Timiskaming volcanics of the Kirkland Lake area reveal three distinct yet intimately associated suites within a composite volcanic-sedimentary sequence. The initial phase of Timiskaming volcanism was characterized by calc-alkaline lavas, which are transitional between the later alkaline volcanics of the Timiskaming and the underlying calcic Keewatin greenstones. Flows of alkalic mafic trachytes and a variety of leucitic lavas and pyroclastic rocks are the latest differentiates represented. This occurrence of leucitic volcanics is the first reported anywhere in the Precambrian. Deposition of sedimentary rocks and intrusions of syenite and mafic syenites were contemporaneous with the period of volcanism, but intrusions of feldspar porphyry were post-Timiskaming.The major mineral constituents are augite, biotite, plagioclase, leucite, and their alteration products. Potash feldspar and hornblende occur in subsidiary amounts, and magnetite and apatite are the common accessory minerals. The chemical composition of this volcanic sequence is marked by high alkalies, Al2O3, and low SiO2, P2O5, and TiO2. The calc-alkaline and alkaline lavas are rich in Na2O and poor in K2O and BaO, whereas the leucite lavas are high in K2O and BaO, and comparatively low in Na2O.The Kirkland Lake sequence is compared with similar exotic, but less altered, lavas of more recent age in order to clarify further the nature of Timiskaming volcanism. The alkalic association of the Kirkland Lake area is similar to those occurring at a late stage in the volcanic cycle of modern volcanic arcs and along orogenic belts marginal to major continental masses.