How are igneous rocks classified by tectonic setting?

Answer.  Some geologists consider tectonic setting to mean a "convergent plate boundary type setting".  Accordingly, extensional settings such as rift zones are called anorogenic environments, as are inplate (within-plate or intraplate) environments.  Since magmas generated at inplate environments are formed above isolated hotspots, and because rift zones are produced at hotspots that lie along a line, the former are also called incipient rifts by some.
 
 

Geological environments at which igneous rocks are formed.
A)    Inplate:
    A1) Continental
    A2) Oceanic
B) Extensional setting
    B1) Rift valleys--Divergent plate boundaries
        B1i) Continental rift valleys
        B1ii) Oceanic rift valleys
    B2) Pull apart basins -Transform plate boundary
    B3) Extension zones  associated with back arc environments adjacent to Convergent plate bondaries, invlves no lateral change of subducted slab

C) Compressional setting.---Convergent plate boundary.
    C1) mantle wedge
    C2)  partially melting upper plate
    C3) partially melting of subducted plate,  -- Adakites.
    C4) Slab window- Window created at change in dip of subducted plate.

A) Inplate environments (incipient rifts.)

(A1) Inplate, Contintental areas

Volcanics

Flood basalts -Tholeiites (Trap series of Deccan Plateau, India; Columbia River Plateau , p.109)

Rhyolites and rhyolite-basalt associations (Yellowstone National Park-p.1326-134; and Newberry, Oregon).

Alkali basalts  (Black Hills, South Dakota; Devils Tower, Wyoming; and Highwood Mountain, Montana)

Plutonics

Carbonatites and alkaline rock complexes, Magnet Cove, Arkansas)

Layered gabbroic bodies- LIPs. (See also Table 9.1, p. 162 of text.) (Bushveld Complex, South Africa; Skaergaard Intrusion, Greenland;  Stillwarter Complex;  Montana; Duluth Complex; Minnesota; Muskox Intrusion of Northwest Territories, Canada; Rhum Complex of Scotland).

Details: Inplate Continental

The Columbia River Plateau Basalt. It stretches from Portland,Oregon, through the Cascades , through Washington to Idaho and Wyoming,  The Snake River and Columbia River expose the basalt, largely quartz tholeiite.  Four groups of basalts (p.109): and Imnaha (17 Ma), Picture Gorge, Grand Ronde (thickest), and Wanapaum Basalt (15 Ma). Volcanism continued  until 6 Ma.

Mg# = 0.30-0.65; Sr ratio shows systematic variation from lower to higher values,from the older Imnaha and Picture Gorge (= 0.703- .7045) to the youngest Saddle Mountain formation (= 0.715), and epsilon Nd value decreases from 8 to -12, from old to young (p.110). These data support mantle melt, ascent of magma, crystal liquid fractionation and assimilation of continental crust.

Passage of the Yellowstone hotspot beneath a back arc setting are possible triggers of the flood basalts.
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Other examples:  flood basalts and layered gabbroic intrusions.

The Trap series of the Deccan Plateau of India formed as India moved over the Reunion hotspot.  Similarly, the Rajmahl was formed as India traversed over the Kerguelen hotspot. The Paran of Brazil, the Bushveld Complex of South Africa, the Skaergaard Intrusion of Greenland, the Stillwater Complex of Montana (p.176); the Sudbury Intrusive of Canada. - all of these formed at inplate sites. Most of them tap magmas generated at sub-asthenospheric mantle. When such deep seated magmas ascend they may cause partial melting of other rocks surrounding their conduit. These rocks are mafic to ultramafic in composition.

The Bushveld is 2 billion years old, Hawaii is modern, the Karoo of South Africa, the Deccan of India, etc., are in between in age. These rocks have tapped the mantle, and we could learn about changes in mantle composition through the years by observing differences in the chemistry of these rocks.

The overall shape of gabbroic layered intrusions is a lopolith (pp.162, 169). Three layering types are present: 1)  rhythmic layering characterized by dark olivine rich layers alternating with light plagioclase rich layers (p.170),  2) uniform layering that exhibits no color banding, and 3) Cryptic layering that is identified by changes in the Na/Ca ratio in plagioclase and Fe/Mg ratio in olivine's.  They also exhibit cross bedding (p. 178).  Generally, (pp. 179, 175) an ultramafic (peridotite, bronzitite zone) is overlain by a banded series identified as lower, middle and upper banded series.  The lower bands contain norite and gabbronorite zone, the middle series contain anorthosite and the upper by gabbronorite.  Generally, chromite band, platinum and palladium bands are found in the ultramfic and lower banded series.  The gabbroic rocks are dominated by cumulus textured minerals (p.179).
 
 

Rhyolite volcanism of the Yellowstone National Park.
Late Cenozoic eruption overlie unrelated Eocene Absaroka volcanics (p. 131).  Absaroka volcanic rocks are overlain by Huckleberry Ridge Tuff, then by Lava Creek Tuff, and in turn by the central member of Plateau Rhyolite (p.133, Fig. 7-7). Three major calderas, produced by supervolcanic eruption, the youngest centered at the Old Faithful and the others to the west of it were vent areas for the rhyolite groups. Milder precaldera volcanism, followed by explosive eruption that yielded tuff units, followed by caldera-forming collapse and then postcaldera eruptions.  The three supervolcanoes are dated 2.0 Ma, 1.2 ma, and 0.6 Ma.  Judging by the interval of these eruptions, another supervolcano is due soon.  A 10 Ma supervolcano from Bruno de Ja Ridge in southwestern Idaho left a 2 meters thick ash in Orchard, Nebraska, 1600 Km away, and suffocated all biota, as indicated by meris (lung disease) that covered all the skeletons of that kill.  Supervolcanoes are devastating.

There is a bimodal distribution of rhyolite and basalt (p. 132).

Rhyolites are white to pink with sanidine and quartz phenocrysts. Pumice and tuff are also present.

Basalts are dark and olivine bearing.

REE show Eu depletion for rhyolite, and enrichment of LREE compared to HREE, and slight positive anomaly  for basalt. p.128.
 
 

Mantle plume is suggested by presence of enriched  ³He/⁴He ratio, and low Mg# in the basalts , and magmatic fractionation by the Eu depletion and REE pattern in rhyolite. Sr ratios range from .709 to .727, and indicate crustal contamination.  Actualy antaxis of crust for the rhyolites is a more likely origin for rhyolites as indicate by Sr ratios.

Whereas the Yellowstone volcanics young to the northeast there is the associated Newberry volcanics, of Oregon that young to the northwest.  Humphreys et al. (2000) suggest that these are comagmatic and might have been produced by "magma propagation" instead of being "plume tracks.
 
 
 

 Alkali basalts and carbonatites.

Apparently, when an aulacogen or old graben passes over hotspots, the lithosphere is weakened and alkali basalts and carbonatites ascend to the surface or are intruded into the weakened crust.

Cox and Van Arsdale (1997) report that the Mississippi Embayment passed over the Bermuda Hotspot 90 MA. Significant uplift, a reactivation of the Paleozoic Pascola Arch over the embayment axis accompanied mid Cretaceous magmatism, prior to latest Cretaceous subsidence.  Alkali plutons of Magnet Cove, Arkansas, and surrounding regions were cooled rapidly (apatite fission track study), and were exposed to erosion, before they were unconformably overlain by basal Upper Cretatceous sedimentary rocks.  Rifted continental margin at Charleston, South Carolina, passed over the Bermuda Hotspots in latest Creatceous.

We might refer to the inplate occurence at Magnet Cove as alkali rocks of the 2nd kind in order tto distinguish them from alkali rocks of the 1st kind, which form during continetal rifting.

Yet other alkali rocks, which we might call alakali rocks of the 3rd kind, also occur at inplate location. Examples of these include the Eocene igneous rocks at the Black Hills of South Dakota, at Devils Tower of Wyoming, and at the Highwood Mountains of Montana. The potassic alkali rocks possiblly result from delamination magmatism - of the sort that involved foundering and sinking in subjacent asthenosphere with attendeant rising of the asthnesphere to fill the space left by the foundering rocks(O'Brien et al., 1991).  Partial melting of the foundering rocks might yield mafics, some of which,depending on the appropriatness of the source-rock mineralogy,  might be silica undersaturated  (alkali rocks), while partial melting of upper level rocks could yield silica saturated rocks. 

Precambrian metasedimentary and metaigneous rocks and granites were uplifted during the Laramide Orogeny into a north-south elonagated elliptical dome that is exposed in western South Dakota and eastern Wyoming.[The Mount Rushmore National Memroial is curved on the Precambrian Harney Peak Granite.] Both silica saturated and undersatuarted rocks were formed as intrusions in South Dakota, Wyoming and Montana. Alkali rocks (phonolites) are exposed in the northern Black Hills east of Spearfish Canyon. West of the Black Hills, in Wyoming, the columnar jointed "mammoth rock obelisk" at Devils Tower and, about 5 miles northwest of it, the Missouri Buttes are possible apophysis or tongues of a laccolith hiden 1000 or more feet below(USGS Bull. 1021-1).  North, in Montana, the alkali field includes the Highwood Mountains (McCollum et al., 1989). At the northeastern foothills of the Highwood Mountains, near  Geraldine, a laccolith of shonkinite sandwiching a syenite is  exposed (Westland, pdf on the web). Reportedly, the rocks formed by the differentiation of on magma.

 [Many examples of alkali rocks are present  in other anorogenic, or non tectonic sites. Also, while carbonatites are limited to continental settings, other types of alkali rocks are also found in oceanic environments.  For example, the Hawaiian islands are constructed of rocks including alkali basalt's.  In all cases, alkali basalts tap deep seated rocks compared to tholeiites for example.)

Uncommon rock types are associated with alkali basalts and carbonatites.  These include: Ijolite (nephelene foidite); tephrite; nephelinite;  Shonkinite (mafic syenite); lamprophyre (containing euhedral mafic minerals); jacupirangite (nepheline-bearing clinopyroxenite); fourchite (aphanitic-porphyritic-with nepheline and augite phynocrysts and a groundmass of garnet, magnetite, sphene, nepheline,, and biotite, tinguaite (pheniocrysts of aegerine and nepheline in a groundmass of kspar, sodalite, calcite, mgnetite); phonolite with kspar, plagioclase and nepheline and other accessory minerals; kimberlites, which are ultramafic rocks found in stable cratons, and contain megacrysts, phenocrysts, and xenocryts of olivine, and phlogopite, garnet, pyroxenes, or ilmenite in a groundmass that include calcite, serpentine and chlorite, (- sometimes brecciated pipes called diatremes are found in kimerlites and may contain diamond); carbonatite with more than 50% carbonate minerals).

Minerals in these rocks include; feldspathoids (nepheline, leucite, sodalite, etc.), alkali pyroxenes (aegerine, etc.), alkali amphiboles (barkeivekite, arfvedsonite, etc), carbonates (calcite, dolomite, ankerite, etc.), biotite, magnetite, and sphene.

In general, tholeiites and alkali basalts are formed in extensional setting or in inplate setting. Carbonatites are limited to continental environments and are formed above hotspots.  They are associated with alkali basalts. (Carbonatites have wt.% SiO₂ less than 45.  Yet, mafic minerals are not abundant.  For this reason some geologists refer to them as ultrabasic and not untrmafic rocks.)

(A2) Inplate, Oceanic areas

Hawaii-  See page 104 text, also Churnet'ss field trip photographs.  Epsilon Nd (9-2 to 9), 87-86 Sr ratio (0.7030 to 0.07046), Mg# = 0.47 to 0.80; some alkali basalts have as low as 0.20.  Alkali basalts have high LREE and low HREE. Alkali basalts formed by low degree partial melting of mantle and from a derivative magma.

Kerguelen and Reunion islands are islands formed above hotspots .  Trap basalts were formed as India traversed over these hotspots.

(Use a Fenner diagram of oxides plotted against MgO (-the FAMOUS example) to show comagmatic oceanic rocks.  The Hawaiian Island Chain are good examples containing both tholeiites and alkali basalts.  Alkali basalts are derived from  the mantle deeper than 80 km, whereas tholeiites are from shallow depth.)

Alkali basalt and tholeiitic rocks and minor rhyolites (Hawaiian island)

(B & C) Plate boundaries are either EXTENSIONAL or COMPRESSIONAL regions.

Note:  Igneous rocks that formed at extensional regions may be obducted or fault emplaced at convergent boundary zones.  Such is the case of dismembered ophiolites that may be present at suture zones.

B) Extensional setting

 B1) Rift valleys--Divergent plate boundaries

B1i) Continental rift valleys

Examples: CONTINENTAL: Rio Grand Rift, Red Sea-East African Rift.

Volcanics

Bimodally distributed tholeiitic basalts and rhyolites  (all rifts)

Tholeiites at rift axis and alkali basalts at rims (e.g., Rio Grand)-

Tholeiites. Alkali basalts and carbonatites (e.g. East African Rift)

Tholeiites of ocean floor rocks (MORB)

Alkali basalts and carbonatites.
 Modern analogs for the formation alkali rocks include East African Rift . In a carbonatite magma, the lava is white as nahcolite (NaHCO3) crystallizes on the surface.
 
 

Peralklaine rocks of rift valleys might result from the "plagiocalse effect".  Since calcian plagioclase has twice as much alumina as alkaline-plagioclase, removal (early formation of calcian plagioclase) will remove deplete alumina from the residual melt, make it alkai rich and thereby produce peralkaline rocks.

Plutonics

Gabbros, syenites, granites (in different proportions at all continental rifts)

Layered intrusions (Palisades Sill, New York and New Jersey, Red Hill Dike of Tasmania, Great Dyke Rhodesia).
Gabbroic dikes or sills might be simple injections charcterized by baked zone of contact, chilled margin of igneous body (e.g., Salisbury Craigs, Edinburgh).  They may consist of multiple injections,  Or they could be differentiated sills as in Palisades, USA. A  sill or dike of  diffentiating magma would how chilled, margin. cumulate textrued basal part. gabbroic middle part and a granophyric top part, the laater perhaps due to high degree of undercooling (see p. 174 the Jurassic Red Hill dike of Tasmania).
 
 

 Origins of alkali basalts and tholeiites of continetal rift.  They tend to be more akin to OIBs than to MORBS

Both alkali basalts and tholeiites are found at rifts.  Volcanic rock suites are comonly bimodal, consisting of basalts and rhyolites.  Generally they are not derived from primary magma. Their chemistry includes,  Mg#  = 0.66-0.47, enriched in LREE (indicative of low degree of partial melting), Sr ratio = 0.7037-0.7050, and Nd ratio is about 0.51243 (indicative of mantle source).  Generally, tholeiites are present at the rift axis and alkalis on the rims. The Rio Grand is described in the text book on p.114.

Low degree of partial meting of mantle results in producing basalts, the tholeiitic variety if melted at shallow depth, and the alkali basalt variety if melted at greater depth.   As magma ascends into the crust, conatmination will provide additional silica content.

Ophiolite complexes (p. 164, 180-185):  Ultarmafics overlain by gabbros, overlain by gabbroic sheeted dikes, overlain by pillow basalts and caped by pelagic sediments.

Example: Trudos Massif, Cyprus; the Jurassic Del Puerto Ophiolite of Coast Range, Central California.
 

 OCEAN FLOOR SPREADING CENTERS: (MORB, THOLEIITES, OPHIOLITES)

The asthenosphere ascends at oceanic spreading centers and cools to form Mid Oceanic Ridge Basalts (MORB), which are known as ophiolite sequences. Regions which are enriched by high outporing of lava are called  EMORBs.  MORB is generally formed by the mixing of two magmas, one of them from a deeper source than the other.  This can be determined from magnesium number (ranging from 0.30- 0.76) and other chemical signature.  Generally MORBs are low in K₂O (<0.15-0.25, low Sr ratio (0.702-.705), Nd ratio (0.5128 to 0.5134), and low Ba, Pb, and Zr.  The oceanic rocks are tholeiites. However, part of the oceanic rock is not pristine MORB, which formed at oceanic ridges. It might have formed as a result of collision of adjacent oceanic plates.  Basically, such recycled ophiolite is more silicic, and has less MgO than pristine MORB.  Locally, differentiation of either recycled or pristine MORB can yield felsic rock (M-type granites).
 

Note that oceanic water (steam) circulates through the sealfloor rock during crystallization.  Thus, ophiolites, are hydrothermally metamorphosed (metasomatized) and altered during their formation.  Accordingly, basalts are altered to spilites (with albitized plagioclase, chloritized pyroxene and amphibole); trachytes are altered to keratophyres (sodium and water metasomatized); ultramafic rocks are similarly metasomatized and altered to serpentinite and talc. Commonly, pillow basalts that characterize the upper stratigraphy of an ophiolite, are pillow spilites which contain crisscrossing fractures that are filled by zeolites and calcite.  Note that an ophiolite is a layered series consisting of, from bottom to top, ultramfic rocks, gabbro, gabbroic dykes, and pillow basalts. (Steinmann trinity includes radilarian chert, pillow basalt and serpentinte - the chert is not part of an ophiolite sequence.)
 
 

B2) Pull apart basins -Transform plate boundary

Both divergent (at pull-apart basins) and convergent (at restraining bends) conditions are present at a transform plate boundary.  Igneous rocks are formed from magmas, which are generated by partial melting at a shallow depth within the lithosphere, that  may ascend at pull-apart basins.

Rhyolites and basalts of the Salton Sea, California, are an example.

 B3) Extension zones associated with convergent plate boundaries. Slab suction pulls on the upper plate and produces back arc rifts.

Back arc environments and regions of extension proximal to convergent boundaries, that have subsequently become zones of extension such as the Basin and Range Province of southwestern USA.

Tholeiitic and alkali basalts of  the Colorado Plateau at the San Franciscan Field of Arizona , such as Sunset Crater,  SP Crater , and of Utah such as  the dykes of Capitol Reef, and in the Basin and Range such as the baslts at Lake Mead, rhyolites and andesites of the Hoover Dam,Nevada;  the St Geroges basalts of Utah,the rhyolitic tuffs and ash falls of Resting Sping, California were all formed in a backarc extensional environment.

At Lake Mead, work by Daley and DePaole (1992) has showed the following. Before extension began (6 Ma) Epsilon Nd of alkali basalts was -9,  whereas, it was +6.4 after extension (4.6 Ma).  Tholeiites, form rocks 10 to 6 Ma, have a range of -10 to -3 epsilon Nd values. Since +ve value represents mantle source, the lithosphere was thined by extension so that the mantle is closer to the surface by 4.6 Ma.  Yet, about 9 Ma, the crust was not thinned enough as tholeites (33-50 Km) were derived by partial melting of the lithosphere.

The Basin and Range Province is characterized by low angle, west -vergent  denudational faults.  The Death Valley, bounded on the west by the Panamint Range and on the east by the Black Mountains is formed by such listric normal faults.   Cenozoic magmatism centered around Black Mountains has yielded flows and falls in the region.  Phreatomagmatism has created  explosion craters as the Ubeheb Crater in Death Valley.

Denudational faults (similar to the Turtlebacks on the Black Mountain of Death Valley) are now detected on the flanks of Mid Oceanic Ridges in the deep ocean.

    Notice that igneous rocks of the Basin and Range Province, although located in a backarc position to the Mesozoic Sierras, are formed in the upper Cenozoic.

In Costa Rica backarc igneous rocks are dominated by alkaline rocks (Abraits and Worner, 2001).
 
Use spider diagrams (normalized to primitive mantle, or to Chondrites) to distinguish back arc rift basalts from other types.  The Pumkinvine Creek formation (Otto Fm) of the Dahlonega Belt, Georgia, has been interpreted to have formed in a backarc setting. [Christi Emery is working on this project.]

C) Compressional setting.---Convergent plate boundary.

    Examples: Andean type:Tuolumna Intrusive Series of the Sierra Nevada Batholith, California; Yosemite National Park, California;   Stone Mountain, Georgia.

    Island arc type: The Tonga-Kermadec Islands, the Aleutian Islands; Cascade Range of Northern California, Oregon, and Washington, the caribbean island, Costa Rica and Panama of Central American .
 

 In general, calcalkaline rocks are formed at volcanic arcs in a compressional setting from magmas that sampled the manlte wedge; Tholeiites  may also be prsent, and alkali basalts are  present in the backarc seting,  adakaites are formed by partial melting of the subducted slab, and others tap magmas from the slab window at which subduction dip changes.

2). CONVERGENT PLATE BOUNDARY.

a) OCEANIC-CONTINENTAL PLATE COLLISIONS. (arc is dominated by intermediate  rocks that plot on a calcakaline trend on AFM diagram: the trend includes calcalkaline basalts (high alumna, 17 to 20 wt.%, compared to tholeiites which have low alumina), andesites, dacites, rhyolites.  Tholeiites may be presentc).  Volcanics are phenocrystic, and plagiocalse shows oscillatory zoning (due to kinitic factor, convection, or magma mixing).

Origin of intemediate magmas:
 

Under conditions of  water-saturated partial melting of mantle wedge, plagiocalse would produce sufficient alumina in the melt that could yield high alumna basalts (calcalkaline basalts.)   Dehydration of subducted lithosphere furnishes the necessary water.
 

Partial melting of mantle wedge saturated with water vapor yields a calklkaline magma, which then mixes with magma that results from partial melting of the lithosphere.  Such mixed magma would be the source of abundant intermediate rocks.

In  ANDEAN TYPE OROGEN, the rocks ar dominated by inermediate and felsic varieties. The volcanic superstructure is dominated by andesite, with some rhyolite, whereas the plutonic substructure is dominated by diorite, granodiorite and granite (I or S-type felsic plutons).

In the western USA, the Sierra Nevada Mountains were formed this way. The magnificent Yosemite Granite (Which was later  sculpted by Pleistocene glaciation) was formed above a subduction zone during the Laramide Orogeny. In the eastern North America, granites of Stone Mountain, Georgia, the Carolinas, Virginia, Pennsylvania, New York, Massachusetts, New Hampshire, and New Fundland, Canada were produced at collisional boundaries and subsequently translated to their current location by tectonic displacement.

In a continental versus continental plate collision, obduction of the oceanic lithosphere places ophiolites on continental rocks. This is useful in identifying ancient convergent plate boundaries. Dismembered or complete ophiolites are found at sutures.

Examples from east of the Hayesville Thrust Fault (an interpreted Taconian Suture) and proximal to Chattanooga include:

 (i) The Pumpkinville Creek Formation, which outcrops on the roadcut along I-75, near Allatoona Lake, Georgia. It contains metamorphosed pillow basalts. The pillows were formed in bodies of water above a backarc setting during the Ordovician (481 Ma- David  et al., 2001).  The pillows were cold-emplaced during the Taconian Orogeny (423-478 Ma) and metamorphosed in the Alleghanian (some 254-318 Ma)

 (ii) Serpentinzed ultrmafics are present where US Highway 64 crosses Chunky Gal, North Carolina. Just east of it, at Buck creek.  Further east, at Winding Stair Gap ultramfic rocks are present in high  grade, granulite facies rocks.

 Note: Andesite line separates basalts of oceanic rocks from andesitic and more felsic rocks of continents.

b) OCEANIC-OCEANIC PLATE COLLISION. (MAFIC TO INTERMEDIATE, RECYCLED MORB)

This process produces island arcs. On the basis of seismogram interpretations,  three types of island arcs  are distinguished.  They are characterized by (i) frontal accretion, (ii) basal accretion, or (iii) erosion were identified. Accretion refers to material scraped off the lower plate and added to the upper plate, whereas erosion refers to removal of materials from the leading edge of the upper plate.

Each of them yields an ARC OROGEN with four distinct lithotectonic provinces: forearc ridge, forearc basin, volcanic arc (magmatic arc), backarc. Obducted MORB (pristine or recycled) may be present in the forearc ridge either as frontal accretion or basal accretion. Partial melting above a subduction zone yields basaltic to andesitic volcanic superstructure, and a substructure composed of gabbros, diorite and granodiorite. These form the central rib or volcanic massif of an island arc. Volcanic and plutonic rocks range from tholeiitic near the forearc, high aluminum basalts (calcalkaline basalts with Al₂O₃= 17 to 20 wt.%) at the volcanic arc, to alkali olivine types toward the backarc, and correspond to depths from which the magmas originated.

Examples for this would be the Tonga-Kermadec island arcs north of New Zealand, the Caribbean islands of Central America. Surrounding the Caribbean Sea, the Caribbean islands in the east andthe Central American islands of Panama and Costa Rica in the West, were formed as island arcs.

In Costa Rica, in the south, subduction of the Cocos Ridge has uplifted the Talamanca Mountains from which the volcanic superstructure is removed by erosion, thereby exposing the plutonic substructure. In northern Costa Rica, however, the volcanism is still active, and pyroclastics are shed which occasionally blanket the forearc basin.
 

c) TRANSITIONAL FROM OCEANIC-OCEANIC TO OCEANIC-CONTINENTAL PLATES. (INTERMEDIATE TO FELSIC)

The Tonga-Kermedic type island arcs and those of the Maraiana provide examples of pure ocean -ocean collision. These are dominated by  basaltic rocks.

The Kuril islands and Aleutian islands, formed by the Pacific seafloor subducting beneath the Okhotsk and Bearing sea, respectively, provide an excellent example of transitional type orogen. They are dominated by basaltic andesites and andesites. Going  north from mariana, through Kuril and from west to east across the Aleutians, the island arcs merge with Andean type orogens of Alaska, through Canada to the Northwestern Territories of the USA (The Cascade Range, p.152).

The volcanic edifices of the Northwestern Territories of the USA., such as Mount Shasta, St. Helens, Crater Lake and others are examples of volcanoes formed in an Andean type orogen. They are composite or stratovolcanoes, composed of alternating flows and pyroclastics of felsic rocks.

The Costa Rican example started out as Kermadec type arc orogen, i.e., orgen produced by oceanic-oceanic plate-collision but continued as transitional type.

        C4)  Slab window- Window created due to a lateral change in dip of subducted plate, from high angle to low angle subducting plate

Go to Question & notes

Cox, R. T., and Van Arsdale, R. B., 1997.  Hotspot origin of the Mississippi Embayment and its possible  impact on contemprary seismicity, Engineering Geology,  V. 46, p. 201-216.
 

Humphreys, E. d., Dueker, K. G, Schutt, D. L., and Smith, . B,  2000. Beaneath Yellowstone: Evaluating plume and nonplume models using teleseismic images of the Upper mantle, GSA Today, v. 10,  0 12, p. 1-7.

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At any rate, there are igneous rock suites that formed at INPLATE  environments and others at PLATE BOUNDARIES.   (Use tectonic discrimination diagrams to distinguish one rock suite from another.   Pearce & Cann, 1973, 1976, for  OFB = Ocean floor basalt, LKT  = low tholeiitic basalt, CAB = calcalkaline basalts,  WFB = inplate basalt;  Ikeda, 1990 for BAB = back arc basalts from Ce-Sr-Sm Diagram.  OIB oceanic island basalt is enriched in ³He/⁴He compared to MORB (mid ocean ridge basalt).  Note that OFB  = MORB; WFB includes OIB (oceanic island basalt); E-MORB (enriched MORB or OIB type source is determined from (Nb/Zr) > 0.1 (Abraits and Worner,2000).

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