Calc-alkaline Basaltic Andesites Research Articles

Plutonism in the southern Abitibi Subprovince; a tectonic and petrogenetic framework

Studies of plutonic rocks in Archean terranes such as the southern Abitibi subprovince have been an important component in the development of an accretionary tectonic model for the Superior province. In the southern Abitibi subprovince, pretectonic ( approximately 2.75-2.70 Ga), low K, tholeiitic calc-alkaline, dioritic to tonalitic plutons (e.g., Bourlamaque batholith; Flavrian pluton) are associated with volcanism and are interpreted to be derived from both partial melting of hydrated mantle during oceanic arc formation and fractionation of tholeiitic to calc-alkaline basalt-andesite. Pretectonic, synvolcanic tholeiitic intrusions (e.g., Dundonald sill, Kamiskotia gabbroic complex: 2.72-2.70 Ga) show in situ fractionation trends, and locally, have mixed magma textures with contemporaneous calc-alkaline granitoids. They are believed to be derived from a midocean ridge basalt (MORB)-like depleted mantle, with little evidence for contamination by an older crustal component. Pre- to syntectonic ( approximately 2.72-2.69 Ga) porphyry and tonalite plutons (e.g., Timmins porphyries, Round Lake batholith) have geochemical characteristics typical of Archean tonalite-trondhjemite suites considered to be derived from melting of basaltic rocks at high metamorphic grades. Late to post-tectonic (<2.69 Ga) diorite-monzonite and syenite plutons (e.g., Watabeag batholith, Lamaque stocks, Otto stock) have large ion lithophile element-enriched calc-alkaline to sho-shonitic compositions. These intrusions and associated Timiskaming-type volcanic rocks are attributed to melting of large ion lithophile element-enriched mantle in the late stages of subduction. Late to post-tectonic biotite-muscovite granite intrusions contain a significant contribution of evolved crust (e.g., S-type characteristics of the Preissac-Lacorne batholith).In the southern Abitibi subprovince the tectonic setting of mantle- and crust-derived intrusions is broadly comparable to magmatic processes in Phanerozoic arcs and accreted terranes. The plutonic rocks record a transition from pre- to synaccretionary subduction-related plutonic suites to late to postaccretionary suites derived from enriched mantle and crustal sources. This framework provides a basis for understanding mineral deposits associated with intrusive rocks of the southern Abitibi subprovince which range from Ni-Cu sulfides in layered synvolcanic tholeiitic intrusions (e.g., Kamiskotia gabbroic complex, Ontario) to volcanogenic sulfides in early tholeiitic and calc-alkaline sequences (e.g., Flavrian pluton, Noranda district, Quebec) to the spatial association of gold with late large ion lithophile element-enriched intrusions (e.g., Lamaque plugs, Val d'Or district, Quebec).

Geology and K-Ar dating of the Tuxtla Volcanic Field, Veracruz, Mexico

The Tuxtla Volcanic Field (TVF) is located on the coast of the Gulf of Mexico in the southern part of the state of Veracruz, Mexico. Volcanism began about 7 my ago, in the Late Miocene, and continued to recent times with historical eruptions in ad 1664 and 1793. The oldest rocks occur as highly eroded remnants of lava flows in the area surrounding the historically active cone of San Martin Tuxtla. Between about 3 and 1 my ago, four large composite volcanoes were built in the eastern part of the area. Rocks from these structures are hydrothermally altered and covered with lateritic soils, and their northern slopes show extensive erosional dissection that has widened preexisting craters to form erosional calderas. The eastern volcanoes are composed of alkali basalts, hawaiites, mugearites, and benmoreites, with less common calc-alkaline basaltic andesites and andesites. In the western part of the area, San Martin Tuxtla Volcano and its over 250 satellite cinder cones and maars produced about 120 km3 of lava over the last 0.8 my. A ridge of flank cinder cones blocked drainage to the north to form Laguna Catemaco. Lavas erupted from San Martin and its flank vents are restricted to compositions between basanite and alkali basalt. The alignment of major volcanoes and flank vents along a N55°W trend suggests an extensional stress field in the crust with a minimum compressional stress orientation of N35° E. In total, about 800 km3 of lava has been erupted in the TVF in the last 7 my. This gives a magma output rate of about 0.1 km3/1000 year, a value smaller than most composite cones, but similar to cinder cone fields that occur in central Mexico. Individual eruptions over the last 5000 years had volumes on the order of 0.1km3, with average recurrence intervals of 600 years. The alkaline compositions of the TVF lavas contrast markedly with the calc-alkaline compositions erupted in the subduction-related Mexican Volcanic Belt to the west, leading previous workers to suggest that the TVF is not related to subduction. Trace-element signatures of TVF lavas indicate, however, that they are probably related to subduction. We suggest that the alkaline character of the TVF lavas is the result of low degrees of melting of a mantle source coupled with a stress regime that allows these small-volume melts to reach the surface in the TVF.

Geochemistry and petrogenesis of Archean mafic volcanic rocks of the southern Abitibi Belt, Québec

The Blake River Group of the late Archean Abitibi Greenstone Belt is characterized by the abundance of differentiated lava series as well as by the presence of coeval tholeiitic and calc-alkaline eruptives. Differentiated tholeiites from this group are interpreted as the product of magma mixing between asthenospheric tholeiites and subduction-related calc-alkaline basaltic andesites. From the lower to the upper southern Abitibi Belt, chemical variations recorded in tholeiites are indicative of a major change in the geodynamic regime. Tholeiites from the lower volcanic sequence were produced by a high degree of partial melting of adiabatically rising diapirs below a rifted lithosphere. Uncontaminated tholeiites from the upper part of the belt reflect a lower degree of partial melting probably due to decreasing lithospheric stretching. This may be related to the closure of the rift-related volcanic basins at the begining of the Kenorean deformational events in the Southern Abitibi Belt.

Volcanism in an arc-transform transition zone: the stratigraphy of the St. Clare Creek volcanic field, Wrangell volcanic belt, Yukon, Canada

The St. Clare Creek volcanic field in the southwestern Yukon overlies a tectonic transition in the Wrangell volcanic belt between subduction to the northwest in Alaska and transform faulting along the Duke River fault in the southeast. Two large polygenetic volcanic centres dominated the Miocene landscape of the St. Clare Creek field: the 18–16 Ma Wolverine centre and the 16–10 Ma Klutlan centre. The Wolverine centre evolved from a small alkaline shield volcano at 18 Ma, from which alkaline basalts, hawaiites and mugearites erupted, to a larger composite volcano between 18 and 16 Ma composed of transitional basalt, basaltic trachyandesite, trachyte and rhyolite lavas, and pyroclastic rocks. The youngest Wolverine lavas are calc-alkaline basaltic andesites, andesites, and hybrid lavas (transitional–calc-alkaline). This temporal progression from alkaline through transitional to calc-alkaline volcanism is accompanied by a systematic increase in the degree of silica saturation and decrease in Fe/Si, Nb/Y, and P/Y ratios. Klutlan lavas have lower Nb/Y and P/Y ratios and are characterized by an opposite eruption sequence. The earliest Klutlan lavas (16–13 Ma) erupted from a composite volcano and include calc-alkaline andesite, rhyolite, and hybrid trachyandesite lavas, followed by transitional basaltic trachyandesites, trachyandesites, trachytes, and rhyolites. Klutlan vulcanism between 13 and 11 Ma was dominated by basaltic fissure eruptions on the southern flanks of the earlier centre and include early mildly alkaline basalts followed by more voluminous transitional basalts. Volcanism reverted to a more central type of activity between 11 and 10 Ma and includes calc-alkaline dacite lava followed by transitional basaltic trachyandesite, trachyandesite, and trachyte lavas.The volcanic stratigraphy of the St. Clare Creek field and 40Ar/39Ar geochronological data provide the basis for understanding the origin of St. Clare magmas in a regional tectonic context. Early Wolverine alkaline volcanism largely reflects leaky transform faulting, whereas subsequent transitional and calc-alkaline lavas record the onset of subduction-related volcanism at the margins of the then active Wrangell arc. The opposite eruption sequence at the Klutlan centre records the demise of subduction-related volcanism between 16 and 13 Ma, due to northwestward migration of the subducted plate. Upwelling of asthenospheric mantle in place of the subducted slab led to the generation of transitional basalts between 13 and 11 Ma, which resulted in more evolved lavas between 11 and 10 Ma.

Geology of the Deer Butte Formation, Malheur county, Oregon: faulting, sedimentation and volcanism in a post-caldera setting

The Deer Butte Formation accumulated during the middle Miocene in fault-controlled basins in an extensional setting. The basins developed as regional faults asserted influence after eruption of ash-flow sheets and collapse of calderas of the Lake Owyhee volcanic field. The sequences of Hurley Flat, Dry Creek, and Oxbow Basin contain a lower basalt tephradominated unit formed by basalt hydrovolcanism overlain by fine-grained fluvial and lacustrine volcaniclastic sedimentary units. The sequence of Freezeout Creek was deposited in an erosional valley that was incised into older units and cut across the concurrently active Wall Rock Ridge fault zone. The sequence of Hurley Flat and Dry Creek contain alkaline tholeiitic basalt flows and tephra deposits, whereas the sequences of Freezeout Creek and Oxbow Basin contain subalkaline calcalkaline basaltic andesite. The compositional change occurred after local uplift due to faulting along the Wall Rock Ridge fault zone. The youngest unit, well-sorted, medium-grained, muscovite-bearing arkose of the arkose of Dry Creek Buttes, was deposited in a large river that drained westward from source areas in western Idaho. The Deer Butte Formation was deposited between approximately 15 and 12.6 Ma, while basin and range-type faulting dominated regional structural patterns.

Petrology of the Rainy Lake area, Minnesota, USA-implications for petrotectonic setting of the archean southern Wabigoon subprovince of the Canadian Shield

The Rainy Lake area in northern Minnesota and southwestern, Ontario is a Late Archean (2.7 Ga) granite-greenstone belt within the Wabigoon subprovince of the Canadian Shield. In Minnesota the rocks include mafic and felsic volcanic rocks, volcaniclastic, chemical sedimentary rocks, and graywacke that are intrucded by coeval gabbro, tonalite, and granodiorite. New data presented here focus on the geochemistry and petrology of the Minnesota part of the Rainy Lake area. Igneous rocks in the area are bimodal. The mafic rocks are made up of three distinct suites: (1) low-TiO2 tholeiite and gabbro that have slightly evolved Mg-numbers (63–49) and relatively flat rare-earth element (REE) patterns that range from 20–8 x chondrites (Ce/YbN=0.8–1.5); (2) high-TiO2 tholeiite with evolved Mg-numbers (46–29) and high total REE abundances that range from 70–40 x chondrites (Ce/YbN=1.8–3.3), and (3) calc-alkaline basaltic andesite and geochemically similar monzodiorite and lamprophyre with primitive Mg-numbers (79–63), enriched light rare-earth elements (LREE) and depleted heavy rare-earth elements (HREE). These three suites are not related by partial melting of a similar source or by fractional crystallization of a common parental magma; they resulted from melting of heterogeneous Archean mantle. The felsic rocks are made up of two distinct suites: (1)low-Al2O3 tholeiitic rhyolite, and (2) high-Al2O3 calc-alkaline dacite and rhyolite and consanguineous tonalite. The tholeiitic felsic rocks are high in Y, Zr, Nb, and total REE that are unfractionated and have pronounced negative Eu anomalies. The calcalkaline felsic rocks are depleted in Y, Zr, and Nb, and the REE that are highly fractionated with high LREE and depleted HREE, and display moderate negative Eu anomalies. Both suites of felsic rocks were generated by partial melting of crustal material. The most reasonable modern analog for the paleotectonic setting is an immature island arc. The bimodal volcanic rocks are intercalated with sedimentary rocks and have been intruded by pre- and syntectonic granitoid rocks. However, the geochemistry of the mafic rocks does not correlate fully with that of mafic rocks in modern are evvironments. The low-TiO2 tholeiite is similar to both N-type mid-ocean-ridge basalt (MORB) and low-K tholeiite from immature marginal basins. The calc-alkaline basaltic andesite is like that of low-K calc-alkaline mafic volcanic rocks from oceanic volcanic arcs; however, the high-TiO2 tholeiite is most similar to modern E-type MORB, which occurs in oceanic rifts. The conundrum may be explained by: (1) rifting of a pre-existing immature arc system to produce the bimodal volcanic rocks and high-TiO2 tholeiite; (2) variable enrichment of a previously depleted Archean mantle, to produce both the low- and high-TiO2 tholeiite and the calc-alkaline basaltic andesite, and/or (3) enrichment of the parental rocks of the high-TiO2 tholeiite by crustal contamination.

Geochemistry, petrogenesis, and tectonic implications of central High Cascade mafic platform lavas

Petrographic and multi-element geochemical data for a 37-sample suite representing Oregon9s central High Cascade mafic platform allow the distinction of basalts ( 2 ) from two types of basaltic andesites with associated high-silica basaltic andesites (53–62 wt % SiO 2 ). All units yield fractionated REE and incompatible element-enriched patterns characteristic of continental tholeiitic magmas. Mount Washington (MW)-type basaltic andesites are distinguished from North Sister (NS) types by relatively higher total REE and other LILE abundances, although lithologic similarities prevail. Trace-element signatures of both types do not support their derivation from magmas represented by basalts, but represeint near-primary melts from regions apart from the sources of primary basalt magmas. Normal basalts are modeled with 14% incongruent partial melting and concomitant olivine fractionation of spinel Iherzolite upper mantle in the presence of water-rich fluids derived from a subducted oceanic slab. Basaltic andesites are modeled as 6% to 12% partial melting of fluid-enriched spinel Iherzolite accompanied by olivine (± clinopyroxene ± plagioclase) fractionation. Unusual units, including one with ground-mass biotite and rare quartz xenocrysts, probably resulted from divergent processes of crustal assimilation, crystal fractionation, or melting from alternate sources. Early Pleistocene eruptions were largely diktytaxitic basalt outpourings in response to crustal readjustments and subsidence which occurred prior to 4.5 m.y. ago. The transformation to extensional tectonics abruptly reduced the amount of andesitic and silicic volcanism and allowed tapping of deeper, mafic magma source regions. Chemical and petrographic data support the concept of juxtaposed tectonomagmatic types such that, extension-related basalts erupted within the realm of calc-alkaline basaltic andesites and their silicic derivatives.

Magma chamber below Poàs volcano, Costa Rica

Direct field evidence for links between volcanic rocks and contemporaneous subvolcanic intrusions are difficult to establish. Such links might, however, be inferred by making physical measurements on a youthful active volcano. Poás is a small composite volcano (elevation 2700 m, diameter 20 km) which has evolved to its present form by a sequence of caldera- and crater-forming episodes, possibly during the last 5 × 10 4 y. There has been a high level of historic activity from the active crater, which is 1 km in diameter and 300 m deep, and which contains a hot lake. Lavas and dominant pyroclastic rocks exposed in the active crater calc-alkaline basaltic andesites and andesites. Poás volcano is therefore typical of those in island arcs and young continental margins. Gravity measurements show that the volcano has a regional negative Bouguer anomaly, of amplitudes 100–200 gu, upon which is superimposed a closed positive anomaly, in the active crater area, about 2 km in diameter and with a maximum amplitude of 100 gu. This is thought to indicate a 1 km radius cylinder of solid rock of basaltic andesite or andesite composition with a shallow upper surface and extending to a depth of several kilometers. The concept of a large shallow magma chamber, with a diameter similar to that of the volcano, appears not to be appropriate for this volcano.