Chapter 10 Igneous r ock a ssociations

10.1 Igneous rock associations 264 10.2 Divergent plate boundaries 264 10.3 Convergent plate boundaries 268 10.4 Intraplate magmatism 283

et al., 2001 ) favoring Archean (> 2.5 Ga) onset 10.1 IGNEOUS ROCK ASSOCIATIONS and others (Hamilton, 1998, 2003 ; Stern, The purpose of this chapter is to relate igneous 2005, 2008 ; Ernst, 2007 ) proposing Protero- rock associations to a petrotectonic frame- zoic initiation of deep subduction ∼ 1 billion work, incorporating information presented in years ago (1 Ga). If the latter is true, then over Chapters 7 – 9 . Petrotectonic associations are 75% of Earth’ s magmatic history occurred suites of rocks that form in response to similar under conditions that pre - date the onset of geological conditions. These associations most modern plate tectonic activity. In addition to commonly develop at divergent plate bounda- questions regarding magmatism at Precam- ries, convergent plate boundaries and hotspots brian plate tectonic boundaries, Phanerozoic (Figure 10.1 ). Hotspots can occur at litho- intraplate magmatism may or may not be sphere plate boundaries (e.g., Iceland) or in infl uenced by lithospheric plate boundaries intraplate settings (e.g., Hawaii). Fisher and (Hawkesworth et al., 1993 ; Dalziel et al., Schmincke (1984) estimate the percent of 2000 ). So while the plate tectonic paradigm magma generated at modern divergent, con- is very useful, it does not address all igneous vergent and hotspot regions as 62, 26 and rock assemblages produced throughout 12%, respectively. Earth’ s tumultuous history. In the following While plate tectonic activity plays a critical sections we will address major igneous petro- role in the development of petrotectonic asso- tectonic associations, beginning with diver- ciations, it is not the sole determining factor. gent plate boundaries. For example, the earliest onset of modern plate tectonics continues to be debated, with 10.2 DIVERGENT PLATE BOUNDARIES some researchers (Kusky et al., 2001 ; Parman Decompression of the asthenosphere in response to lithospheric extension results in Earth Materials, 1st edition. By K. Hefferan and partial melting of mantle peridotite at diver- J. O’Brien. Published 2010 by Blackwell Publishing Ltd. gent margins. Basic (basaltic) melts rise and IGNEOUS ROCK ASSOCIATIONS 265

Continental arc Island arc Divergent convergent convergent ocean Backarc boundary boundary ridge basin

Hotspot

Figure 10.1 Major tectonic environments where igneous rocks occur. (Courtesy of the US Geological Survey and US National Park Service .) solidify to produce oceanic crust, while refrac- peridotite towards the bottom of the section, tory residues cool below a critical tempera- marking the base of ocean crust. The ture to form the thickening mantle layer Mohorovi č i ć discontinuity (Moho) occurs at of ocean lithosphere. Ocean lithosphere is the contact between cumulate rocks in layer created primarily at spreading ridges such as 3 and non - cumulate, metamorphosed rocks in the Mid - Atlantic Ridge, East Pacifi c Rise and layer 4, marking the rock boundary between Indian – Antarctic ridge systems. A small per- the ocean crust and mantle. Layer 4 is com- centage of ocean lithosphere is generated in posed of depleted mantle peridotite refractory backarc basin spreading ridges (e.g., Maria- residue (e.g., harzburgite, dunite). Layer 4 nas Trough) and ocean hotspots (e.g., Hawaii). mantle peridotite is marked by high tempera- In all cases, anatexis of ultramafi c mantle ture, solid state strain fabric (metamorphosed) is the primary magmatic source of ocean and represents the lowest layer of the oceanic lithosphere. lithosphere. Ocean lithosphere contains four distinct Layers 3 and 4 are generally unexposed on layers as indicated in Figure 10.2 a. Layer 1 ocean fl oors because they are overlain by contains well - stratifi ed marine pelagic sedi- layers 1 and 2. In rare locations, these deep ments and sedimentary rocks that accumulate layers are exposed on the ocean fl oor in ultra on the ocean fl oor. Layer 2 can be subdivided slow (< 1 cm/yr), or magma- starved, spreading into two basaltic rock layers. An upper layer ridges and transform zones where brittle contains pillow basalts that develop when faulting and uplift processes bring them to the basic lavas fl ow onto the ocean fl oor, rapidly surface. Slices of ocean lithosphere are also cool in the aqueous environment and solidify preserved in alpine orogenic belts as ophiolite in spheroidal masses (Chapter 9 ). Beneath sequences. Let us now consider petrotectonic the pillow basalt pile, basic magma injects assemblages that form at ocean ridge spread- into extensional fractures producing steeply ing centers. inclined diabase dikes as the magma cools and contracts. Repeated horizontal extension and 10.2.1 Mid - o cean r idge b asalts magma intrusions generate thousands of dikes arranged parallel to one another in a sheeted At ocean spreading centers (Figure 10.2 b – c), dike complex (Chapter 8 ). Beneath the sheeted partial melting of lherzolite (peridotite) gener- dike layer, basic magma cools slowly, allow- ates voluminous, geochemically distinct, ing phaneritic crystals to nucleate and grow mid - ocean ridge basalts (MORB) and gabbros as layer 3. Layer 3 contains massive (iso- containing minerals such as plagioclase, tropic) gabbro in the upper section, layered augite, hypersthene, pigeonite, diopside and (cumulate) gabbro in a middle section, and olivine. MORB are the most abundant increasing amounts of layered (cumulate) volcanic rocks on Earth. Typical major and 266 EARTH MATERIALS

(a) Layer 1 0 Sediment Pillow basalt Layer 2 Basaltic dikes 2 Oceanic crust 4 Gabbro Layer 3

6 Layered gabbro

Layered peridotite Moho 8 Mantle Layer 4 Peridotite

10 km below sea floor (b) Rift zone Depth Pillow basalts (km) Sheeted dikes 0 1 2 3 Magma 4 Gabbro chamber Crystallization 5 Rising magma

(c) Ridge crest Oceanic crust Depth (km) 20 40 Oceanic lithosphere 60 80 Magma source region Asthenosphere

(d)

Juan de Fuca Ridge

M id - At lan tic Equator

Ridge Rise Central Indian Ridge

Pacific

East Southeast Indian Ridge

Ridge Southwest Indian Ridge

Pacific Antarctic IGNEOUS ROCK ASSOCIATIONS 267

Table 10.1 Trace element abundances for N - MORB and E - MORB in parts per million (ppm). (After Best, 2003 ; data from Sun and McDonough, 1989 .) LIL HFS LREE HREE Cs Rb Ba Th U Nb Ta La Ce Pr Nd Sm Zr Eu Gd Yb Lu

N - MORB 0.007 0.56 6.3 0.12 0.47 2.33 0.132 2.5 7.5 1.32 7.3 2.63 74 1.02 3.68 3.05 0.455 E - MORB 0.063 5.04 57 0.6 0.18 8.3 0.47 6.3 15 2.05 9 2.6 73 0.91 2.97 2.37 0.354

E - MORB, enriched mid- ocean ridge basalt; HFS, high fi eld strength; HREE, heavy rare Earth elements; LIL, large ion lithopile; LREE, light rare Earth elements; N - MORB, normal mid- ocean ridge basalt.

minor element concentrations are indicated in Rock/chondrite 1000 Table 10.1 . MORB are low SiO2 (45– 52%), low potassium ( < 1% K2 O) tholeiites with high MgO ( ∼ 7 – 10%), Al2 O3 (15 – 16%) and compatible element concentrations (Ni and 100 Cr ∼ 100– 500 ppm). MORB develop from E-MORB partial melting of a depleted mantle source, as indicated by low 87 Sr/ 86 Sr ratios (0.702– 0.704), low volatile and incompatible element 10 concentrations, and high compatible element concentrations (Cann, 1971 ). “ Depleted N-MORB source ” refers to mantle lherzolite that has 1 undergone previous melt cycles that largely La Ce Nd Sm Eu Tb Tm Yb Lu removed mobile incompatible elements (Chapter 7 ). Figure 10.3 Chondrite- normalized rare Earth Mid - ocean ridge basalts can be subdivided element patterns for enriched and normal into normal MORB (N- MORB) and enriched mid - ocean ridge basalt (E- MORB, squares; MORB (E- MORB) based upon minor and N - MORB, triangles) samples collected from trace element abundances (Figure 10.3 ; Table the Mid- Atlantic Ridge. (From Schilling et al., 10.1 ). N - MORB are strongly depleted in 1983 ; with permission of the American highly incompatible elements such as large Journal of Science .) ion lithophile (LIL) elements (such as Cs, Rb and Ba), high fi eld strength (HFS) elements (such as Nb and Ta) and light rare Earth ele- ments (LREE, such as La, Ce, Pr, Nd and Sm). These geochemical characteristics imply that N - MORB magma represent 20– 30% partial melting of a well - mixed, depleted mantle source (Frey and Haskins, 1964 ; Gast, 1968 ). Although the major element and heavy rare Earth elements (HREE, ranging from Eu to Figure 10.2 (a) Idealized stratigraphy of Lu) concentrations are comparable, E - MORB ocean lithosphere and ophiolites. Note the have higher incompatible element (LREE, petrological Moho between layers 3 and 4, HFS, LIL) concentrations relative to N- MORB. separating the base of the crust from the Specifi cally, E - MORB are defi ned by having upper mantle. (Courtesy of the Ocean Drilling chondrite normalized La/Sm ratios of > 1. Project.) (b) Block diagram of ocean ridge Lanthanum may occur in concentrations of divergent margins. (c) Ocean ridges are 1 – 5 ppm in N- MORB but up to 100 ppm in primary sites for the generation of ocean E - MORB. lithosphere. (d) The global distribution of How can we account for chemical varia- divergent margins. tions between N- MORB and E- MORB? 268 EARTH MATERIALS

Several different hypotheses have been pro- Convergent margin magmatism may occur posed. First, E - MORB may represent smaller for thousands of kilometers parallel to the degrees (∼ 10 – 15%) of partial melting of trench, and up to 500 km perpendicular to the residual mantle rock so that the incompatible trench in the direction of subduction (Gill, elements are more highly concentrated in 1981 ). Plutonic rocks at convergent margins E - MORB magmas. Second, E- MORB could include diorite, granodiorite, quartz diorite, be tapping a deep mantle source that has granite, gabbro, tonalite and rocks referred to not been previously melted. Third, E - MORB as trondhjemite. This plutonic suite of rocks could represent magma enriched from magma occurs in batholiths above subduction zones mixing, assimilation or partial melts derived and provides magma to overlying volcanic from subducted ocean lithosphere. For arcs. The spectrum of possible volcanic rock example, Eiler et al. (2000) , based on a study types varies widely from youthful island arc of 28 basalt samples from the Atlantic, Pacifi c environments – dominated by arc basalts and and Indian ridges, propose that E - MORB basaltic andesites – to mature continental arc include a component of partially melted systems – comprised largely of andesites, with oceanic lithosphere that has been recycled lesser amounts of basalt, dacites, rhyodacites into the upper mantle from ancient subduc- and rhyolites. tion zones. As opposed to relatively simple decompres- While MORB is the dominant volcanic sion melting of the mantle at divergent rock type at divergent margins, other rock margins, convergent margin magmatism is types occur in varying proportions. Ocean affected by more variables, each of which can ridges also produce high aluminum basalts, diversify magma composition. These varia- where the Al2 O3 concentrations are > 16%. bles include: Other rocks such as andesite, icelandite, fer- robasalt, trachyte, hawaiite, mugearite, tra- • Composition (continental versus oceanic) chybasalt, trachyandesite, dacite and rhyolite and thickness of the overlying converging can occur as minor components at ocean plate: thinner ocean lithosphere in the ridges as well as in “ leaky ” transforms, con- overlying plate generally produces metalu- tinental rifts and ocean islands. The andesitic minous, mafi c to intermediate rocks. to rhyolitic volcanic rocks at ocean ridges Thicker continental lithosphere overlying have higher TiO2 (> 1.3%) concentrations the subduction zones commonly yields compared to more common convergent peraluminous, potassic, intermediate to margin varieties and are always subordinate silicic rocks. to basalt (Gill, 1981 ). • Composition of rock material experienc- Divergent margins (see Figure 10.2 d) gener- ing anatexis: Earth materials experiencing ate the bulk of ocean fl oor rocks, which repre- partial melting may include overlying sent ∼ 70% of Earth’ s area. As a result, the ultrabasic mantle wedge, basic to silicic ocean ridge basalt and underlying gabbro and forearc basement, subducted basic – peridotite are widespread in our relatively ultrabasic ocean lithosphere, and marine young ocean basins, all of which are less than sedimentary material. The relative propor- 200 million years old. What happens to old tion of each of these components affects ocean lithosphere? At least for the past 1 billion the composition of plutonic and volcanic years, it has been subducted and recycled at rocks generated in the arc system. convergent margins, as discussed below. • Flux melting whereby volatile- rich miner- als such as micas, amphiboles, serpentine, talc, carbonates, clays and brucite release 10.3 CONVERGENT PLATE H O, CO or other volatile vapors that BOUNDARIES 2 2 lower the melting temperature of mantle While divergent plate boundaries are domi- peridotite and eclogite (high pressure nated by MORB, chemically diverse igneous metagabbro) overlying the subduction assemblages erupt in the convergent margins zone. widely distributed in the Pacifi c Ocean, eastern • Diversifi cation processes such as fraction- Indian Ocean and the Caribbean and Scotia ation, assimilation and magma mixing Seas (Figure 10.4 ). (Chapter 8 ) as well as metamorphic reac- IGNEOUS ROCK ASSOCIATIONS 269

OF

Aleutian trench Kurile trench FIRE RINGJapan trench Izu Bonin trench Ryukyu trench Philippine Puerto Rico trench trench Marianas trench Middle America Challenger Deep trench Equator Bougainville trench

Java (Sunda) trench Tonga trench Peru-Chile trench

Kermadec trench

South Sandwich trench

Figure 10.4 Earth ’ s convergent margins. (Courtesy of the US Geological Survey.)

tions (Chapter 15 ) strongly alter magma subduction may not have been possible composition generated in the overlying (Davies, 1992 ; Ernst, 2007 ; Stern, 2008 ). wedge of the arc system. This may explain the origin of some unique • Dip angle of the subduction zone wherein Archean rock assemblages as well as the old, cold, dense lithosphere favors steep virtual absence of Archean blueschists dis- subduction and young, warm, buoyant cussed later in this text. lithosphere produces shallow subduction zones (Figure 10.5 ). Steeply inclined sub- While magma composition is highly variable duction zones allow for the melting of based on the factors described above, Phan- thick wedge- shaped mantle slabs in the erozoic convergent margins are dominated by overlying plate. Shallowly dipping sub- the calc- alkaline suite of rocks whose chemis- duction zones allow only thin wedge - try is enriched in SiO 2 , alkalis (Na2 O and shaped mantle slabs to intervene above the K2 O), LIL, LREE and volatiles and is rela- subducting plate, minimizing overlying tively depleted in FeO, MgO, HFS and HREE mantle wedge input. The negative buoy- concentrations (Miyashiro, 1974 ; Hawkes- ancy of old, cold, dense ocean lithosphere worth et al., 1993 ; Pearce and Peate, 1995 ). is the key force driving deep lithosphere The presence of hydrous minerals such as subduction in modern plate tectonics over hornblende and biotite indicates that arc the past 1 Ga. This negative buoyancy may magmas contain > 3% H2 O. Volatiles play not have been present in the hot, buoyant an important role in subduction zone fl ux Archean ocean lithosphere such that deep melting. 270 EARTH MATERIALS

MARIANAS-TYPE MARGIN CORDILLERAN-TYPE MARGIN

Backarc Island Ocean Continental basin arc ridge arc

Young Old lithosphere lithosphere Steep Low angle subduction subduction

Figure 10.5 The steeply dipping Marianas- type island arc subduction model and the shallowly dipping Cordilleran continental arc subduction model. Note the thick, deep, mantle wedge overlying the Marianas - type margin and the thin mantle wedge in the Cordilleran model.

The calc - alkaline association of basalt, tholeiites are also referred to as high alumi- andesite, dacite and rhyolite (BADR) is the num basalts. The calc- alkaline basalts differ signature volcanic rock suite of convergent from tholeiites in having higher alkali (notably margins and constitutes one of the most volu- K2 O) concentrations and not displaying iron minous rock assemblages on Earth, second enrichment typical of tholeiitic fractionation only to MORB (Perfi t et al., 1980 ; Grove and trends (Figure 10.6 ). As discussed in Chapters Kinzler, 1986 ). Harker diagram plots of major 7 and 9 , magma viscosity and explosiveness elements (see Figure 8.11 ) generally indicate are proportional to SiO 2 increases. As a result, a liquid line of descent from a common source, the more siliceous volcanic rocks described such that BADR rocks are derived from a below commonly produce pyroclastic tuff and common parent magma of basaltic composi- breccia deposits in addition to aphanitic to tion. Andesite, named for South America ’ s aphanitic– porphyritic crystalline textures. Andes Mountains, which overlie the Peru – Andesites are volcanic rocks containing Chile trench, is by far the most common calc - > 52 – 63% SiO2 . Andesites can be subdivided alkaline volcanic rock forming at convergent based upon the range of SiO 2 : basaltic andes- margins (see Figure 10.4 ). The more silicic ites, common in youthful island arc systems, (dacite, rhyolite) members of the BADR group contain > 52 – 57% SiO2 while more silicic represent more highly fractionated daughter andesites, common in mature continental arc products. We will discuss each of these below. systems, contain > 57 – 63% SiO2 (see Figure The major rock types in volcanic arc 7.24 ). Andesites commonly occur as gray, systems can be distinguished based upon porphyritic– aphanitic volcanic rocks with major element concentrations such as SiO 2 , phenocrysts of plagioclase, hornblende, K 2 O and Al 2 O3 content. Basalts contain pyroxene or biotite. Plagioclase phenocrysts 45 – 52% SiO2 and can be subdivided into a are most common and may display euhedral, number of different varieties based upon zoned crystals. Hornblende phenocrysts are major and minor element concentrations. also common and may display reaction rims. Basalts common in convergent margins Pyroxene (principally augite, hypersthene or include aphanitic and aphanitic – porphyritic pigeonite) and biotite phenocrysts less varieties of arc tholeiites (low K 2 O) and calc- commonly occur. Quartz, potassium feldspar, alkaline basalts (moderate K2 O). Plagioclase olivine or feldspathoid phenocrysts are rare. phenocrysts are common. The arc tholeiites Interestingly, the bulk composition of andes- differ from other tholeiitic basalts (MORB ite (and its plutonic equivalent diorite) and ocean islands) in containing higher con- approximates that of terrestrial crust, suggest- centrations of Al 2 O3 , typically in concentra- ing that subduction zone processes have tions greater than 16 wt %. As a result, arc played a signifi cant role in the development IGNEOUS ROCK ASSOCIATIONS 271

(a) F = FeO + 0.9Fe2O3 of continental crust (Hawkesworth and Kemp, 2006 ). The generation of voluminous andesite is favored by subduction angles greater than ∼ 25 ° , anatexis of thick (greater than ∼ 25 km), continental, hanging wall plates and partial melting of subducted slabs at depths of 70 – 200 km (Gill, 1981 ). Dacites (Chapter 7 ) are quartz– phyric vol- MORB canic rocks, intermediate between andesite and rhyolite (Gill, 1981 ). While most dacites contain 63– 68% SiO2 , the total alkali to silica Mg-rich Ol & Px ( TAS) dacite classifi cation extends to 77%

A = Na2O + K2O M = MgO SiO2 (see Figure 7.24 ). Dacites are enriched (b) F = FeO + 0.9Fe O in plagioclase and are the volcanic equivalent 2 3 of granodiorites, in which alkali feldspars are subordinate to plagioclase. When present, phenocrysts are commonly subhedral to euhe- Fe-enrichment trend dral, zoned and generally consist of oligoclase to labradorite plagioclase or sanidine. Minor minerals commonly include biotite, horn- blende, augite, hypersthene and enstatite. Calc-alkaline trend Trachyandesites (also known as latites Calc-alkaline and shoshonites ) are generally composed Andesites basalts of ∼ 66 – 69% SiO2 , although the lower TAS Rhyolites Dacites Mg-rich Ol & Px limit begins at 57% SiO 2 (see Figure 7.24 ). Trachyandesites commonly contain phenoc- A = Na2O + K2O M = MgO rysts of andesine to oligoclase plagioclase (c) feldspar amidst a groundmass of orthoclase K O wt% 2 High-K 4 and augite. Rhyolites ( > 69% SiO 2 ) and rhyodacites Dacite and rhyolite ∼ 3 ( 68 – 73% SiO2 ) are associated with explo- Basaltic andesite 68, 3.1 sive silicic eruptions producing fragmental,

Andesite Medium-K 2 glassy and aphanitic to aphantic – porphyritic

Basalt textures. Rhyodacite is a rock term, not rec- 48, 1.2 1 68, 1.2 ognized by the IUGS system, for intermediate Low-K 48, 0.3 volcanic rocks that bridge the dacite/rhyolite 45 49 53 57 61 65 69 73 77 boundary. These rocks can occur as glasses SiO wt% (obsidian or pumice), pyroclastic tuffs and 2 breccias, or as aphanitic to aphanitic– porphy- ritic crystalline rocks. Common phenocrysts Figure 10.6 (a) Tholeiitic mid- ocean ridge include alkali feldspar or quartz, with minor basalts (MORB) display iron enrichment due concentrations of hornblende and biotite. to the early crystallization of magnesium- rich In addition to variations in SiO , arc rocks olivine and pyroxenes. (b) The calc- alkaline 2 display signifi cant variation in K 2 O concen- suite does not display signifi cant iron trations, ranging from low (tholeiitic), medium enrichment but displays alkali enrichment (calc- alkaline) and high K 2 O (calc - alkaline with progressive crystallization. (c) Three to shoshonite) rock suites (Gill, 1981 ). The volcanic rock suites are recognized on the progression from tholeiite to calc - alkaline to basis of percent Si2 O and KO 2 : low potassium shoshonite (trachyandesite) refl ects increasing assemblages consist of tholeiitic basalt; K2 O and K 2 O/Na2 O and decreasing iron medium potassium assemblages contain enrichment (Jakes and White, 1972 ; Miya- calc - alkaline assemblages; high potassium shiro, 1974 ). K2 O content in convergent suites consists of high potassium calc- alkaline margin volcanic suites broadly correlates with rocks and trachyandesite rocks referred to as the thickness of the overlying slab in conver- shoshonites (Gill, 1981 ; LeMaitre, 2002 ). gent margin systems (Figure 10.6 c). Low 272 EARTH MATERIALS

(a) 10.3.1 Island a rcs

Trench Island arc Ocean lithosphere is subducted beneath an Oceanic crust Continental overlying plate composed of oceanic litho- crust sphere (Figure 10.7 a) producing island arc Lithosphere Lithosphere chains in the eastern Indian Ocean, the Carib- bean and Scotia Seas and the western Pacifi c Asthenosphere Ocean, notably the Marianas Islands. Island arcs develop on the overlying ocean litho- sphere plate, above the subduction zone. Island arc volcanoes are underlain by inter- (b) mediate to mafi c plutonic suites dominated by diorite, quartz diorite, granodiorite, tonalite arc Trench Volcanic and even gabbro. Diorites contain < 5% quartz Oceanic crust Continental crust and quartz diorites contain 5 – 20% quartz (see Figure 7.20 ); both of these rocks are Lithosphere Lithosphere enriched in plagioclase and hornblende, with lesser amounts of pyroxene and biotite. Horn- Asthenosphere blende (and to a lesser degree pyroxene and biotite) imparts dark (mafi c) colors, while the plagioclase tends to result in lighter (felsic) Figure 10.7 (a) Ocean– ocean convergence hues; together, these mineral suites tend to producing island arc volcanoes and backarc occur in approximately equal concentrations, basins. (b) Ocean– continent convergence producing a speckled light and dark colora- producing continental volcanic arcs. (Courtesy tion. Diorite, quartz diorite and granodiorite of the US Geological Survey.) batholiths intrude beneath youthful volcanic arcs. Granodiorites , which represent the plu- tonic equivalent of dacites and rhyodacites, contain > 20% quartz and more plagioclase than potassium feldspar. Island arc granodi- orites are generally metaluminous, containing potassium tholeiites dominate with overlying hornblende, biotite and minor amounts of slab thicknesses ranging from ∼ 0 to 20 km; muscovite. Island arc plutons can also consist medium - to high potassium calc - alkaline of tonalites and trondhjemites, which are plu- andesites are associated with overlying slab tonic rocks enriched in plagioclase feldspar thicknesses of ∼ 20 – 40 km; high potassium and quartz. Tonalites , fi rst described from shoshonites commonly develop where the Monte Adamello near Tonale in the eastern overlying slab is > 40 km thick (Gill, 1981 ). Alps, contain calcium plagioclase and quartz Three major types of convergent margins with minor amounts of potassium feldspar, occur: (1) ocean – ocean convergence generat- biotite and hornblende. Trondhjemites , also ing youthful island arc volcanic complexes known as plagiogranites, are granodioritic (Figure 10.7 a), (2) ocean– continent conver- rocks in which sodium plagioclase represents gence generating mature continental arc half to two - thirds of the total feldspar complexes (Figure 10.7 b), and (3) continent– component. continent convergent margins marked by the In addition to the voluminous calc- alkaline cessation of subduction and consequent con- rock suite dominated by andesites and basal- tinental collision. Many of the same rock tic andesites discussed earlier, young island types can be found in all three environments; arc systems also produce low potassium arc however, continental systems contain a tholeiite basalts as well as relatively rare rocks greater proportion of silicic calc - alkaline named boninites and adakites. Low potas- rocks enriched in quartz and potassium sium arc tholeiites occur on the oceanward feldspar; in contrast, island arcs contain a side of the volcanic arc, nearest the trench. greater proportion of mafi c to intermediate Tholeiitic magmas commonly form at sub- rocks as described below. duction zones where the overlying plate is IGNEOUS ROCK ASSOCIATIONS 273 relatively thin. Major element concentrations by slab melting of eclogite and/or garnet of the tholeiitic island arc basalts are very amphibolite from the descending ocean litho- similar to MORB, as indicated by their sphere (Kay, 1978 ; Defant and Drummond, relatively low K 2 O concentrations and iron 1990 ; Stern and Killian, 1996 ; Reay and Par- enrichment, suggesting a similar depleted kinson, 1997 ). While it was initially believed mantle source – most likely by fl ux melting of that adakites only form where young the ocean lithosphere wedge overlying the ( < 25 Ma), thin, hot ocean lithosphere is subducted slab as well as the subducted slab subducted beneath island arc lithosphere, itself. Island arc tholeiite basalts can be dis- adakites are now known to form at conti- tinguished from MORB by greater concentra- nent– continent collision sites as a result of tions of potassium and other LIL elements shallow slab subduction of continental litho- (such as Ba, Rb, Sr, Cs, Rb and U) and lower sphere (Chung et al., 2003 ). Shallow slab concentrations of HFS elements (such as Th, subduction and lithosphere recycling at sub- Hf, Ta, Ti, Zr, Nb and Y) (Perfi t et al., 1980 ; duction zones may play a signifi cant role in Hawkesworth et al., 1993 ; Pearce and Peate, the development of adakites as well as their 1995 ). Tholeiitic island arc magmas com- plutonic equivalents trondhjemites and tonal- monly produce basalts, basaltic andesites and ites. Research continues to determine possible andesites in the volcanic arc and diorite, relationships of adakite formation with tonalite (plagiogranite) or lesser granodiorite Archean tonalite, trondhjemite and granodi- plutons in the underlying magmatic arc. orite (TTG) associations and the evolution of Boninites , named for the Bonin Islands in continental crust (Drummond and Defant, the western Pacifi c Ocean, are high magne- 1990 ; Castillo, 2006 ; Gomez- Tuena et al., sium (MgO/MgO + total FeO > 0.7) interme- 2007 ) (Box 10.1 ). diate volcanic rocks that contain a Insofar as the overlying arc lithosphere is SiO 2 - saturated (52– 68% SiO2 ) groundmass. relatively thin in immature island arc systems, These rare rocks contain phenocrysts of young volcanic arcs are dominated by basalts orthopyroxene, and notably lack plagioclase and basaltic andesites with rare boninites phenocrysts (Bloomer and Hawkins, 1987 ). and adakites. Prolonged subduction in island Boninites are enriched in chromium (300 – arc systems generates increasingly thicker arc 900 ppm), nickel (100– 450 ppm), volatile lithosphere. As island arc lithosphere thick- elements and LREE as well as zirconium, ens, andesites and dacites predominate as the barium and strontium. Boninites are depleted Si2 O and K2 O contents of all rocks increase in HREE and HFS elements. These unusual with the development of continental - type arc rocks occur proximal to the trench and bear lithosphere (Miyashiro, 1974 ). In nearly all the geochemical signature of primitive convergent margins, the calc - alkaline associ- mantle- derived magmas produced early in ation is generated by fractional crystalliza- the subduction cycle (Hawkins et al., 1984 ; tion of basaltic magma derived by partial Bloomer and Hawkins, 1987 ; Pearce and melting of overlying mantle peridotite – Peate, 1995 ). Thus, boninites are a product fl uxed by fl uids released from the dehydrated of subduction - related melting in the forearc subducted oceanic lithosphere slab. The con- of youthful island arc systems. Van der Laan tinued removal of crystals from melt leads et al. ( 1989 , in Wyman, 1999 ) suggest that to continuous variation in the residual liquid boninites are produced by high temperature, (liquid line of descent) generating basalt, low pressure remelting of previously sub- andesite, dacite and rhyolite. In addition to ducted ocean lithosphere. Interestingly, bon- fractionation, open- system diversifi cation inites can be associated with rare ultrabasic processes (Chapter 8 ) such as assimilation komatiites that we will discuss later in this and magma mixing alter magma chemistry chapter. (Grove and Kinzler, 1986 ) and alter the Adakites are silica - saturated (> 56% SiO2 ) chemical composition of magmas generated rocks with high Sr/Y and La/Yb ratios (LREE in the mantle wedge overlying the subducted enriched relative to HREE) and low HFS slab. (such as Nb and Ta) concentrations. Adak- In addition to magmatism within the island ites, named for Adak Island of the Aleutian arc complex, igneous activity can also occur Island chain, have been thought to be derived behind the island arc in backarc basins. 274 EARTH MATERIALS

Box 10.1 Tonalite, trondhjemite and granodiorite (TTG ) association

Plutons containing TTG are found in subduction zone environments ranging from the Archean to the Recent. However, Archean (> 2.5 Ga) subduction zone plutonic rocks consist dominantly of TTG. Trondhjemites were named by V. M. Goldschmidt in 1916 for holocrystalline, leucocratic Norwe- gian rocks enriched in sodium plagioclase and quartz and depleted in biotite and potassium feldspar (Barker, 1979 ). Trondhjemites are similar to tonalites but contain greater concentrations of sodium plagioclase (oligoclase to albite) and more variable potassium feldspar concentrations (Figure B10.1 a). Tonalites and trondhjemites are also known as plagiogranite. Charnockites , an orthopyroxene - bearing suite of rocks of generally granitic composition, also occur with the TTG association. TTG associations occur in Archean rocks such as the Pilbara Craton of Australia, and the Bear- tooth and Big Horn Mountains of Wyoming. In contrast, Proterozoic and younger convergent margin granitic rocks consist predominantly of granite and granodiorite, with TTG associations representing a very small component in ocean – ocean convergence. What conditions changed at ∼ 2.5 Ga? Shallow - dipping subduction zones “ pinch out” the overlying mantle wedge such that the wedge component plays a relatively minor role in magma genesis. Archean subduction involved higher geothermal gradients, shallow subduction and melting of downgoing ocean lithosphere, with minimal input from the overlying lithosphere wedge. Shallow subduction of oceanic lithosphere at unusually low angles (Figure B10.1 b) has been proposed as a model for the growth of Archean continental crust through the generation of TTG plutonic rocks and their volcanic equivalents – adakites (Martin, 1986 ; Smith- ies et al., 2003 ).

(a) An Figure B10.1 (a) Classifi cation of some granitoid rocks enriched in plagioclase and > 20% quartz. Trondhjemite is a light- colored tonalite containing sodium - rich oligoclase albite. Ab, albite (sodium plagioclase); An, anorthite (calcium plagioclase); Or, orthoclase (potassium feldspar). (b) Shallow subduction of the Archean ocean lithosphere may have produced tonalite plutons and adakite Tonalite volcanic rocks. (Courtesy of the Geological Survey 30% Granodiorite of Canada; with permission of the Natural Resources of Canada 2009.) Granite AbTrondhjemite Or 30%

(b) Adakite “Pinch-out” volcanoes wedge zone

Mantle pinch-out zone “Tonalite factory” due to shallow subduction

Shallow subduction? Refertilized mantle wedge IGNEOUS ROCK ASSOCIATIONS 275

Backarc b asins and volcanic arc (Chase, 1978 ). Extension is manifested as normal faults and backarc Although compressional forces dominate spreading. The western and northern Pacifi c island arc settings, lithospheric extension can Ocean (Figure 10.8 a) provide excellent exam- occur in the overlying plate, behind the arc, ples of backarc basins, including the Sea of resulting in the development of backarc basins Japan, the Bering Sea, the Lau Basin – Havre (Figure 10.8 ). How does backarc extension Trough, Manus Basin and the Marianas occur? “ Trench pull” forces move the vol- Trough. canic arc towards the subduction zone result- Backarc basins (Figure 10.8 b) erupt a ing in the seaward movement of the trench diverse suite of volcanic rocks including

(a) 0° 60°E 120°E180° 120°Ｗ 60°Ｗ

60° Aleutians Tyrrhenian Sea BAB Kurile Okinawa Trough BAB Japan Cascades 30° Izu-Bonin MexicoCentral America Ryukyu Mariana Trough BAB Manus Basin BAB Lesser Andaman New Antilles 0° Britain North Fiji Basin BAB Sea BAB Fiji Atlantic Indian Pacific Solomons Ocean Ocean Tonga Ocean New Lau Basin BAB Andes 30° Hebrides Kermadec Hikurangi E. Scotia Sea BAB (New Zealand) Scotia arc 60° Bransfield South Shetlands Strait

(b) Backarc Volcanic Forearc Accretionary basin arc basin mélange Trench

0 Arc crust 500° 500°C ere Lithospheric osph 1000°C lith mantle ting duc 50 Sub Depth (km)

100

150 400 300 200 100 0 Distance from trench (km)

Figure 10.8 (a) Modern backarc basins (BAB) are concentrated in the western Pacifi c Ocean. (b) Backarc basins form by extension within the arc crust. (Courtesy of Wikipedia.) 276 EARTH MATERIALS basalt, basaltic andesite, andesite and dacite; sources include the hydrated mantle wedge however, tholeiitic and alkalic basalts com- situated above the downgoing slab, recycled monly dominate. As suggested in our earlier subducted lithospheric slab, and subducted discussion of island arc tholeiitic basalts, the marine sediment. Thus, BAB are produced relative proportions of andesitic versus basal- by a combination of partial melting of lher- tic magmas is related to the nature and thick- zolite upper mantle wedge that has been ness of the lithospheric wedge above the fl uxed by volatiles released by the subducted subduction zone (Gill, 1981 ). In some island ocean lithosphere, as well as decompression arc settings (e.g., Kermadec, Marianas, Scotia melting of mantle peridotite at backarc and Vanuatu arcs), basalts dominate over spreading ridges (Fretzdorff et al., 2002 ). andesites in the backarc and the volcanic arc As indicated in the discussion above, distinc- regions. Fryer et al. (1981) , on the basis of tion between different types of basalts gener- trace element chemistry, identifi ed a distinc- ated in different tectonic environments is tive group of rocks known as backarc basin largely dependent upon geochemistry (Box basalts (BAB). BAB are tholeiitic, with geo- 10.2 ). chemical similarities with both MORB and arc tholeiite trends. Relative to MORB, BAB 10.3.2 Continental m argin a rcs display greater enrichment of H 2 O, alkali ele- ments and LIL elements. BAB are slightly Mature convergent margins, involving the depleted in titanium, yttrium and niobium subduction of ocean lithosphere beneath and display fl at rare Earth element patterns thick continental lithosphere, occur along the 5 – 20 times that of chondrites (Sinton et al., eastern Pacifi c region extending from the 2003 ). BAB may show relative enrichment in Cascades southward to the Andes Mountains volatile elements, thorium and LREE, which (Figure 10.8 a). Ascending hydrous melts from suggests the involvement of subduction- the subducted ocean slab chemically react related fl uids in magma genesis (Pearce and with the overlying wedge composed of mantle Peate, 1995 ). and thick continental lithosphere. These Why do backarc basins produce a wide magmas produce continental arc plutons that array of rock types that range from near are more silicic than island arc plutons as a MORB to calc - alkaline compositions? Exten- result of the thick overlying continental litho- sion in the backarc (Figure 10.8 b) results sphere through which subduction zone fl uids in partial melting of mantle peridotite, pro- must penetrate. Extensive assimilation and ducing MORB- like magmas. However, these magma mixing within the overlying continen- magmas interact to varying degrees with tal slab result in K 2 O and SiO2 enrichment calc - alkaline sources. Calc- alkaline magma within plutons.

Box 10.2 Geochemical approaches to petrotectonic associations

Basalt, basalt, basalt! As indicated in the preceding discussion, basalt can be produced in a number of different tectonic environments. Petrotectonic studies utilize a number of different approaches to identifying sites of basalt genesis. One of these approaches utilizes geochemical indicators discussed in Chapter 7 . Pearce and Cann (1973) combined geochemical variations of minor and trace elements to infer tectonic origin. The Pearce and Cann (1973) classifi cation (Figure B10.2 a) is widely used in the tectonic analysis of basalts and is particularly useful in accretionary terranes (Chapter 1 ) where the original source of basalt is ambiguous. The petrotectonic environments defi ned by these discrimi- nation diagrams include: within plate basalts, island arc tholeiites, calc- alkaline basalts, mid- ocean ridge basalts, ocean island tholeiites and ocean island alkaline basalts. Shervais (1982) also developed discrimination diagrams using minor element concentrations such as vanadium and titanium (Figure B10.2 b). These geochemical techniques designed to determine petrotectonic origin are extremely useful if used in conjunction with fi eld studies – and if metamorphism has not chemically altered the rock. IGNEOUS ROCK ASSOCIATIONS 277

Box 10.2 Continued

(a) 1000 WBP: within plate basalts IAT: island arc tholeiites CAB: calc-alkaline basalts MORB: mid-ocean ridge basalts MORB OIT: ocean island tholeiite OIA: ocean island alkaline basalt (ppm) Cr 100 Ti/100 TiO IAT 2

10 OIT 5 10Y (ppm) 50 100 MORB WBP IAT MORB IAT IAT OIA CAB CAB

Zr Y×3 MnO × 10 P2O5 × 10 (b) IAB (159/256 used, 18% error) MORB (197/241 used, 22% error) 1 0.89 500 500 0.8 0.71

0.6 0.53 300 300

V (ppm) 0.4 V (ppm) 0.35

0.2 0.18 100 probability Posterior 100 probability Posterior 0 0 12×104 12×104 Ti (ppm) Ti (ppm) OIB (158/259 used, 9% error) (514/756 used, 16% error) 1 500 500 0.8

0.6 IAB MORB OIB 300 300

V (ppm) 0.4 V (ppm)

0.2 100 probability Posterior 100 0 12×104 12×104 Ti (ppm) Ti (ppm)

Figure B10.2 (a) Discrimination diagrams for basalt using minor and trace element concentrations such as Zr, Ti, Y, Cr, MnO and P2 O5 . (After Pearce and Cann, 1973 ; with permission of Elsevier Publishing.) (b) Discrimination diagrams using vanadium and titanium concentrations as discriminating elements. IAB, island arc basalts; OIB, ocean island basalts; MORB, mid- ocean ridge basalts. (After Shervais, 1982 ; with permission of Elsevier Publishers.) 278 EARTH MATERIALS

Ocean – continent convergent margins sphere. Melting at the base of this lithospheric produce voluminous granodiorite, diorite, stack produces Al2 O3 - , K2 O - and SiO2 - rich granite and tonalite plutons. Large granodi- igneous rocks such as rhyolites, rhyodacites orite plutons commonly dominate ocean – and shoshonites and plutonic rocks of increas- continent convergent plate boundaries. For ingly granitic composition. These magmas are example, the Sierra Nevada consists of 25 – the result of relatively fl at subduction (< 25 ° ), 30 km thick upper crustal rocks containing thick continental lithosphere ( > 25 km), higher hundreds to thousands of individual granodi- degrees of partial melting of continental litho- oritic, dioritic and tonalitic plutons. The sphere and/or arc basement, and the dimin- Sierra Nevada composite batholith developed ished role of ocean lithosphere subduction. due to eastward - dipping subduction beneath Alkaline basalts also occur in continent – western North America. Arc magmatism continent collisions as a result of upwelling continued from 220 to 80 Ma (Fliedner et al., mantle melts. 2000 ; Ducea, 2001 ). Rhyolites and rhyodacites are character- Magma from these intermediate– silicic ized by high viscosity, which retards lava plutons erupts onto Earth’ s surface producing fl ow, resulting in thick accumulations of composite volcanoes. Together with volumi- limited aerial extent. Common minerals nous andesites, rocks such as dacites, include quartz, potassium feldspar, biotite, rhyodacites, rhyolites and latites display aph- plagioclase, anorthoclase and magnetite. anitic – porphyritic to pyroclastic textures in Shoshonitic magmas are generated farthest composite volcanic settings such as Mt St from the trench, wherein melts assimilate K2 O Helens and Crater Lake in the Cascade Range and Na 2 O as they rise through thick slabs (USA). of overlying continental lithosphere. Volcanic In late stages of ocean – continent subduc- eruptions of rhyolite to shoshonite lavas tion, highly alkalic shoshonites can erupt over can erupt explosively, generating voluminous thick continental lithosphere. Shoshonites are pyroclastic tuffs and breccias, or produce lava dark - colored, potassium- rich trachyandesites, fl ows that solidify to produce glassy and/or commonly containing olivine and augite aphanitic– porphyritic textures. phenocrysts with a groundmass of labradorite The plutonic equivalent of rhyodacite and plagioclase, alkali feldspar, olivine, augite and rhyolite are granitic rocks or granitoids (Box leucite. Shoshonites occur in thickened litho- 10.3 ). The term “ granitic ” or “ granitoid ” is sphere farthest from the trench region, in loosely used for silica - oversaturated plutonic continent – continent collisions and in some rocks that contain essential potassium feld- backarc basins. spars and quartz. Granitoids include the IUGS fi elds of quartzolite, quartz- rich granitoid, alkali feldspar granite, granite, quartz grani- 10.3.3 Continental c ollision z ones toid, granodiorite and tonalite (see Figure In long - lived convergent margins, the termi- 7.20 ). Granitoid rocks that form at mature nation of ocean lithosphere subduction marks convergent margins tend to be peraluminous the transition from mature arc– continent con- to metaluminous, containing hornblende, vergence to a fi nal collision of two continental biotite and/or muscovite. Although granites blocks. The Alpine – Himalayan orogenic of variable composition occur, S - type and system is the modern classic model of conti- I - type granites tend to predominate. I- type nent – continent collision following tens of mil- magmas form by partial melting of basic to lions of years of ocean lithosphere subduction. intermediate igneous rocks in or above the Ancient examples include the complete subduction zone at ocean – ocean or ocean– subduction of ocean basins 250 million years continent convergent margins. Peraluminous, ago, resulting in continent– continent collision potassium- rich, S- type granites and granodi- creating the supercontinent Pangea. orites are particularly common at continent – In continent– continent collisions, the lower continent collisions. The peraluminous continental lithosphere does not subduct to sedimentary component is derived from phyl- great depths but essentially breaks off and losilicate minerals in graywackes and mud- underplates the overlying continental litho- stones of the continental crust and accretionary sphere plate producing a doubly thick litho- wedge. These sedimentary materials melt to IGNEOUS ROCK ASSOCIATIONS 279

Box 10.3 Granite classifi cation

Strictly speaking, the term “ granite ” is restricted to plutonic rocks containing 20 – 60% quartz and 35 – 90% alkali to plagioclase feldspars (see Figure 7.20 ). Thus, the two essential mineral groups in granite are quartz and feldspars. Other minor minerals include hornblende, biotite and muscovite. Accessory minerals include magnetite, rutile, tourmaline, sphene, apatite, molybdenite, gold, silver and cassiterite. Granite plutons are genetically associated with Precambrian cratons and convergent margins (Pitcher, 1982 ). Phanerozoic granitic plutonic belts are found along continent– ocean sub- duction zones or at continent – continent suture zones. Within orogenic settings, granites may be emplaced synchronous (syn- kinematic) with convergence, as late- stage collisional plutons or as post- kinematic intrusions. Granites have been subdivided by a number of methods, one of which attempts to infer source rock origin. Chappell and White (1974) and others recognize four distinct types of granite (M, I, S, and A types) based upon the nature of the inferred parental source rock (Table B10.3 ). M- , I- , and S - type granites are orogenic granites associated with subduction, whereas A- type granites are anorogenic in origin.

• M - type granites (Pitcher, 1982 ) are derived from mantle- derived parental magmas, as indicated in the low Sr 87 /Sr 86 ratios ( < 0.704). M - type granites are associated with calc- alkaline tonalites, quartz diorites and gabbroic rocks. In addition to quartz and feldspars, hornblende, clinopyrox- ene, biotite and magnetite are among the major minerals. M - type granites develop in island arc settings. Copper and gold mineralizations are associated with M- type granites. • I - type granites (Chappell and White, 1974 ) are generated by the melting of an igneous protolith from either the downgoing oceanic lithosphere or the overlying mantle wedge. I- type granites are 87 enriched in Na2 O and Ca 2 O and contain lower Al 2 O3 concentrations. I - type granites have Sr / Sr86 ratios of less than 0.708, usually in the range 0.704– 0.706, indicating magma derived from a mantle source. Because they are primarily derived from a mantle source, I- type granites may be enriched in mafi c minerals such as hornblende, biotite, magnetite and sphene. Porphyry copper, tungsten and molybdenum deposits are associated with I - type granites. I - type granites are prevalent along the Mesozoic– Cenozoic Andes Mountains (Chappell and White, 1974 ; Beck- insale, 1979 ; Chappell and Stephens, 1988 ). • S - type granites (Chappell and White, 1974 ) are produced by the melting of sedimentary crustal

rocks in collision zones. S- type granites are depleted in Na2 O but enriched in Al2 O3 (peralumi- nous). S - type granites have Sr 87 /Sr 86 ratios of > 0.708, indicating that source rocks had experienced an earlier sedimentary cycle. S - type granites are also known as two- mica granites in that they commonly contain both muscovite and biotite, refl ecting the peraluminous content of the sedi- mentary source rock rich in phyllosilicate minerals. Hornblende is conspicuous by its absence. Other minerals include monazite and aluminosilicate minerals such as garnet, sillimanite and cordierite. Tin deposits are associated with S- type granites (Chappell and White, 1974 ; Beckinsale, 1979 ).

Table B10.3 The major features of M - , I - , S - and A - type granitoids. M I S A

SiO2 54 – 73% 53 – 76% 65 – 79% 60 – 80%

Na2 O Low, < 3.2% High, > 3.2% Low, < 3.2% > 2.8%

K2 O/Na2 O Very low Low High High Sr87 /Sr 86 < 0.704 < 0.706 > 0.706 0.703– 0.712

Continued 280 EARTH MATERIALS

Box 10.3 Continued

• A - type granites (Loiselle and Wones, 1979 ) are anorogenic rocks produced by activities that do not involve the subduction and collision of lithospheric plates. A - type granites are enriched in alkaline elements with high K/Na and (K + Na)/Al ratios as well as high Fe/Mg, F, HFS elements (Zr, Nb, Ga, Y, Zn) and rare Earth element concentrations. A- type granites are depleted in Mg, Ca, Al, Cr, Ni and have lower water contents and high Ga/Al ratios (Collins et al., 1982 ; Whalen et al., 1987 ). Relative to I - , S - and M - type granites, A - type granites are more enriched in LIL elements and depleted in refractory elements (Creaser et al., 1991 ). A- type granites are peralkaline and commonly contain biotite, alkali pyroxenes, alkali amphiboles and magnetite (Collins et al., 1982 ). Associated with A - type granites are alkali - rich, relatively anhydrous rocks that can include alkali granite, syenite, alkali syenite and quartz syenite. produce two - mica granites containing biotite neous rock assemblage embedded within a and muscovite. highly deformed mud matrix. M é langes form Convergent margins contain a diverse range at subduction zones where rocks and tectonic of rock types. In addition to intermediate and blocks are sliced from the downgoing oceanic silicic igneous rocks prominently discussed lithosphere and often mixed with rocks thus far in this chapter, basic and ultrabasic formed in forearc settings. The diverse suite assemblages also occur at convergent margins of rocks may include: (1) deformed and altered due to magmatic processes and/or tectonic mid - ocean ridge, ocean island and ocean displacement. In some cases, these involve plateau basalts, (2) limestone, chert and other metamorphic processes that we will address marine sedimentary rocks, and (3) slices of in Chapters 15 – 18 . Let us fi rst consider tec- eclogite, peridotite and blueschist from tonically emplaced Alpine orogenic complexes subducted oceanic or forearc lithosphere. and then we will briefl y discuss basic – Eclogites and blueschists are high pressure ultrabasic zoned intrusions. metamorphic rocks characteristic of subduc- tion zones (Chapter 18 ). Ophiolites constitute one type of Alpine 10.3.4 Alpine o rogenic c omplexes deposit in which the oceanic or backarc basin Alpine orogenic complexes are fault- bounded, lithosphere or volcanic arc basement rocks deformed rock sequences that mark the site are preserved in orogenic belts. The term ophi- of present or former convergent margins. olite was fi rst proposed by Steinmann (1905) Unlike the intermediate to silicic igneous for serpentinized rocks in the Alps. Over the rocks that develop in situ (in place) as a result next several decades, Steinmann recognized a of subduction- induced magmatism, alpine suite of rocks that, thereafter, became known orogenic complexes have been transported as the “ Steinmann trinity ” . These three rock far from their site of origin by thrust faulting types consist of pelagic chert, serpentinite and shearing. Because such tectonism can be (hydrothermally altered peridotite) and spilites intense, these complexes are commonly dis- (altered pillow basalts). As the term is cur- membered into fault blocks and jumbled rently used, ophiolites are thought to repre- together in a haphazard fashion such that sent coherent slices of oceanic lithosphere, their original layering may be disrupted. volcanic arc basement or backarc basin litho- Alpine orogenic bodies contain disrupted sphere “ obducted ” , or thrust, onto the edge of pelagic sediment layers, basalt, cumulate basic continents above subduction zones. and ultrabasic layers as well as tectonized While it is intuitively obvious that ophio- mantle slices of ocean lithosphere and calc - lites may originate by sea fl oor spreading at alkaline intrusive and volcanic assemblages. ocean ridges, petrologists recognize that many Alpine orogenic bodies are commonly associ- ophiolites represent oceanic fragments pro- ated with tectonic m é langes. duced in either forearc or backarc settings. A tectonic m é lange (from the French word These ophiolites are referred to as suprasub- for mixture) is an intensely sheared, heteroge- duction zone (SSZ) ophiolites . SSZ ophiolites IGNEOUS ROCK ASSOCIATIONS 281

Iceland Norway Atlin Ballantrae EURASIAN NORTH Bay of Hokkaido Alps Ural AMERICAN Islands Lherz Klamaths PHILIPPINE Thetford Franciscan PERSIAN Cuba Indus PACIFIC AFRICAN ARABIAN Papua Oman SOUTH SOMALI Ecuador AMERICAN NAZCA

Dun ATLANTIC Tasmania Mountain Macquarie Sarmiento INDIAN Island ANTARCTIC DIVERGENT BOUNDARIES -OPHIOLITE CONVERGENT BOUNDARIES

Figure 10.9 Ophiolite locations throughout the world. (Courtesy of William Church.)

develop due to extensional tectonics that While the idealized stratigraphic layering of result in backarc spreading or forearc spread- ophiolites mimics ocean lithosphere layering, ing producing oceanic lithosphere. Research- the complete four- layer stratigraphic sequence ers use immobile elements as petrogenetic is rarely preserved. Tectonically disrupted indicators, such as chromium, to determine ophiolites, missing one or more layers, are ophiolite sites of origin (Dick and Bullen, referred to as partial, or dismembered, ophio- 1984 ). Both the origin of ophiolites and their lites. Ophiolites occur throughout the world means of emplacement on the edges of conti- (Figure 10.9 ) and mark the former location nents remain areas of intense research. of ocean lithosphere subduction. Excellent Complete ophiolite sequences display a examples of ophiolites include the following stratigraphic sequence similar to that of ocean localities: Oman, Troodos (Cyprus), Coast lithosphere (see Figure 10.2 a). The stratigra- Range (California), Newfoundland and phy of an idealized ophiolite sequence was Morocco. Ophiolites are important in provid- defi ned by the fi rst Penrose Conference in ing the following: 1972 as follows: 1 Valuable ore deposits containing Cu, Ag, • Layer 1: pelagic, marine sedimentary rock Au, Zn, Ni, Co, Cr and other metals. such as ribbon chert, thin shale beds and 2 Evidence documenting oceanic lithosphere limestone derived from the lithifi cation of subduction dating from the Precambrian siliceous ooze, clay and calcareous ooze to the present. Well - documented ophio- sediments, respectively. lites less than 1 Ga occur throughout the • Layer 2A: basic volcanic complex, which world. Archean examples, dating as far may contain pillow basalt. back as 3.8 Ga, are highly controversial • Layer 2B: basic sheeted dike complex. and may (Furnes et al., 2007 ) or may not • Layer 3: cumulate gabbroic complex (Hamilton, 2007 ; Nutman and Friend, with basal cumulate peridotites and 2007 ) represent true ophiolites. pyroxenites. • Layer 4: tectonized ultrabasic complex In addition to their occurrence in Alpine consisting of variably metamorphosed orogenic complexes and ophiolites, basic harzburgite and dunite. Podiform chro- and ultrabasic magmas intrude convergent mite deposits occur with dunite bodies. margin assemblages to form the concentri- The tectonized ultrabasic complex overlies cally zoned or layered plutons discussed a metamorphic basal sole thrust. below. 282 EARTH MATERIALS

132° 55’

Clarence Strait

56° 08’

Kashevarof Passage

Cretaceous Igneous Rocks Dunite

N Chromite + PGE

Wehrlite

0510 Olivine clinopyroxenite Kilometers Gabbro hornblende Sulfide + PGE Paleozoic Descon Formation

Figure 10.10 Zoned intrusion from the Blashke Islands Complex, southeast Alaska. PGE, platinum group elements. (After Kennedy and Walton, 1946 .)

Ural Mountains, these plutonic bodies have 10.3.5 Alaska - t ype (z oned) i ntrusions since been identifi ed in many other localities Alaska - type intrusions consist of concentri- throughout the world. Buddington and cally layered (zoned) plutons formed in con- Chapin (1929) noted the concentric layers in vergent margin settings. Alaska - type intrusions the Blashke Island Complex of Alaska (Figure are commonly several kilometers in diameter, 10.10 ). exhibit a dunite core and pyroxenite shell, Irvine (1959) conducted extensive work and are surrounded by massive gabbro. Late on the Cretaceous age Duke Island Complex granitic zones may also occur around the (Alaska). The Duke Island Complex consists perimeter of the intrusive structure. In con- of a 3 km diameter plutonic body at Judd trast to tectonically emplaced alpine suites, Harbor and a 5 km diameter intrusion at Hall Alaska - type plutons form in situ by intrusion Cove. These two intrusions, which are likely of magma into the surrounding country rock. to be continuous at depth, contain a dunite Originally recognized in Alaska and in the core surrounded by successive rings of peri- IGNEOUS ROCK ASSOCIATIONS 283 dotite, olivine pyroxenite, hornblende – 10.4 INTRAPLATE MAGMATISM magnetite pyroxenite and gabbro – all of which are cut by late granitic rocks. Irvine Intraplate magmatism refers to magma gen- (1959) noted layers that exhibited graded eration and igneous rock suites generated beds within ultrabasic rocks at Duke Island, within lithospheric plates, rather than at plate in which crystals are coarse grained at the boundaries. Intraplate magmatism may be base and fi ne upward within a layer. Duke initiated by hotspot activity, continental rifts Island remains an active exploration site with or overthickened continental lithosphere. economic deposits of copper, nickel, platinum Intraplate magmatism produces a wide range and palladium. of igneous rock types including: Alaska - type ultrabasic– basic plutons com- monly occur as post - orogenic intrusions in • Tholeiitic to alkalic basalt and related volcanic arc or accretionary mé lange terrains. gabbros of hotspots and LIP. A number of different processes have been • Siliceous anorogenic granite and rhyolite. suggested for their formation; these include • Silica - undersaturated rocks. fractionation of ultrabasic or basic parent • Basic – ultrabasic suites including komati- magma from the upper mantle, magma mixing ites and kimberlites. at convergent plate boundaries, or magmas • Carbonatites. from deep mantle plumes (Taylor, 1967 ; Tistl et al., 1994 ; Sha, 1995 ; Ishiwatari and Ichi- Large igneous provinces (LIP) , encompassing yama, 2004 ). Alaska- type intrusions are eco- volumes > 106 km 3 (Mahoney and Coffi n, nomically important as sources of metals, 1997 ), are the greatest manifestation of intra- particularly platinum group elements (PGE). plate magmatism on Earth (Figure 10.11 ). Thus far in this chapter we have focused Most LIP are basaltic in composition although upon petrotectonic assemblages from diver- silicic examples, known as SLIP (silicic large gent and convergent plate boundaries. While igneous provinces) , such as Yellowstone, also divergent and convergent margins produce occur. The most widespread Phanerozoic the bulk of Earth’ s magmatism, igneous rocks intraplate magmatic features consist of also develop within lithospheric plates without massive tholeiitic fl ood basalts. These massive any direct link to plate boundary processes, volcanic landforms occur as both oceanic as discussed below. fl ood basalts and continental fl ood basalts .

60° Ontong NAIP Siberian Traps Columbia Basin & Plateau River Range CAMP Emeishan 30° Sierra Ethiopia Madre CAMP Yemen CAMP Rajm Occidental 0° Deccan NW Etendeka Aust Parana E Aust –30° 90° Karoo 90° TVZ Chon Aike Kerguelen –60° Mafic LIP Dronning Maud Land Silicic LIP (intraplate) 0° Silicic LIP (backarc-related)

Figure 10.11 Earth’ s large igneous provinces (LIP). CAMP, Central Atlantic Magmatic Province; NAIP, North Atlantic Igneous Province; TVP, Taupo Volcanic Zone. (After Coffi n and Eldholm, 1994 ; courtesy of M. F. Coffi n.) 284 EARTH MATERIALS

In the following section, we will fi rst melts of an upper, depleted (previously melted) consider oceanic intraplate magmatism and mantle, OIB were considered to perhaps rep- then later discuss continental intraplate resent partial melts from a deeper, undepleted assemblages. mantle source. However, ocean island basalts display large variations in strontium, neo- dymium and lead and other isotopic ratios, 10.4.1 Oceanic i ntraplate m agmatism suggesting the role of multiple sources and Ocean islands and ocean plateaus form above processes. Various hypotheses proposed for mantle hotspots that erupt anomalously high OIB chemistry include: volumes of tholeiitic and alkalic basaltic lava onto the ocean fl oor. Ocean islands are vol- • Small degrees of melting of a primitive canic landforms that rise upward above sea mantle source. level. Seamounts are volcanically produced • Melting of a mantle source enriched in peaks below sea level. Oceanic plateaus are alkali elements. broad, fl at - topped areas that result from • Incorporation of subducted oceanic crust massive outpourings of lava fl owing laterally in the source region. from source vents. Oceanic plateaus cover • Entrainment of subducted sedimentary large areas of the ocean fl oor, ranging up to rocks in the source region (Hofmann 10 million km 2 . and White, 1982 ; Hofmann, 1997 ; Kogiso et al., 1998 ; Sobolev et al., 2005 ). Ocean i sland b asalts The isotopic signatures of many OIB indicate Ocean island basalts (OIB) are a geochemi- that magmas were derived from non- primitive cally distinct suite of rocks distinctly different sources of variable mantle composition. For from MORB (Figure 10.12 ). In contrast to example, Rb/Sr and Nd/Sm ratios are lower MORB, OIB are more alkalic and are less than primitive mantle ratios while U/Pb, Th/ depleted – and may in fact be somewhat Pb and U/Th ratios are higher than primitive enriched with respect to incompatible ele- mantle sources (Hofmann and White, 1982 ). ments such as potassium, rubidium, uranium, In fact, most OIB display isotopic ratios indic- thorium and LREE (Hofmann and White, ative of an enriched mantle source, particu- 1982 ; Hofmann, 1997 ). The different geo- larly their elevated incompatible element and chemical signatures have been interpreted to NiO concentrations. Why is the mantle com- represent different mantle source areas. While position so variable? One explanation involves MORB were considered to represent partial mantle enrichment due to the incorporation of a recycled oceanic lithosphere derived from ancient subduction zones (Hofmann and 30 White, 1982 ; Hofmann, 1997 ; Turner et al., 25 2007 ). Hirschmann et al. (2003) and Kogiso

Phonolites Critical plane of SiO2 et al. (2003) propose that undersaturated, 20 undersaturation

O % nepheline normative OIB magmas may be 2 15 derived by partial melting of a garnet pyrox- Trachytes

O + K enite, which itself is derived by the mixing of

2 Benmoreites 10 Rhyolites Na Mugearites subducted MORB basalt and mantle peridot- Hawaiites ite. Thus, at least some OIB hotspots tap Basalts 5 mantle melts enriched by ocean lithosphere subducted up to 2.5 Ga (Turner et al., 2007 ). 30 40 50 60 70 80 Let us consider the best known OIB location SiO2 % – the Hawaiian Islands. Figure 10.12 The fractionation sequence Hawaii occurring at ocean islands produces a diverse From the ocean fl oor up, the Hawaiian Islands suite of volcanic rocks with variable alkali constitute the highest mountains on Earth and silica concentrations. (Courtesy of with a relief of ∼ 10 km high; Mt Everest in Stephen Nelson.) contrast has an elevation of just over 9 km. IGNEOUS ROCK ASSOCIATIONS 285

The Hawaiian hotspot has been active for processes. While OIB volcanism at locations over 80 million years, generating a chain of such as the Hawaiian Islands is impressive, seamounts and islands extending for a dis- humans have yet to witness the immense vol- tance of 5600 km. The Hawaiian Islands are canism necessary to erupt gargantuan ocean dominated by olivine tholeiites; tholeiitic plateaus. basalts comprise ∼ 99% of the exposed Hawai- ian volcanic rocks with alkalic basalts con- Oceanic fl ood b asalt p lateaus tributing only a small fraction. Early eruptions of alkali basalts are followed by extensive The Ontong– Java Plateau, located near the tholeiitic basalts that generate massive shield Solomon Islands in the western Pacifi c Ocean, volcanoes. Late - stage Hawaiian volcanism is the largest oceanic fl ood basalt plateau on reverts to alkali basalts (hawaiites to ben- Earth. As summarized by Fitton and Godard moreites), perhaps indicating a lower degree (2004) , the dates for the Ontong– Java erup- of melting as the island moves away from the tions are somewhat enigmatic. The Ontong – hotspot magma source and temperatures Java fl ood basalts erupted either in a single decline. The dominant tholeiitic iron - massive fl ood eruption ( ∼ 122 Ma) or in a enrichment trend (Chapter 8 ) likely results series of eruptions spread over 10 million from fractional crystallization of early formed, years with the initial massive outpouring magnesium- rich olivine and pyroxene. As occurring ∼ 122 Ma. The Ontong– Java Plateau magmas become more enriched in iron, alkali- encompasses a surface area of 2 million km 2 enriched hawaiite forms by fractionation of and a volume of 60 million km3 . On the basis tholeiitic basalt. The addition of magnetite to of ten Ocean Drilling Project (ODP) rock core the crystallizing assemblage causes the remain- analyses, the Ontong– Java Plateau region ing magmas to become progressively less is thought to consist largely of a relatively enriched in iron. Continued crystallization homogeneous low potassium tholeiite that produces an iron - depletion and alkali - erupted as massive sheet fl ows and pillow enrichment trend in which magmas evolve basalts, accompanied by minor volcaniclastic successively from hawaiite to mugearite and and vitric tuff deposits (Coffi n and Eldholm, benmoreite (see Figure 7.24 b ). Depending 1994 ; Fitton and Godard, 2004 ). On the basis upon the degree of silica saturation, the fi nal of geochemical data (e.g., enrichment in minerals to crystallize may include feld- incompatible elements Zr to Lu), Tejada et al. spathoids in silica - undersaturated phonolite, (2004) suggest that the Ontong– Java basalt potassium feldspar in silica- saturated trachyte magma was derived by 30% melting of a or quartz in silica- oversaturated rhyolite primitive, enriched, high magnesium (15 – (Figure 10.12 ). 20 wt % MgO) mantle source. Hawaiian basaltic magmas are thought to The origin of oceanic intraplate magma- form by partial melting of a heterogeneous tism continues to be an active area of mantle plume source, perhaps composed of research. At least in some cases, intraplate primitive garnet lherzolite, enriched by a sub- magmatism may be partially derived by deep ducted ocean lithosphere component. Using convection cells involving ancient subducted geochemical data such as Ni, Mg, Pb, O, Hf ocean lithosphere. In other cases, intraplate and Os isotopic ratios, researchers (Blichert - magmatism may be driven by mantle plumes Toft et al., 1999 ; Sobolev et al., 2005 ) propose unrelated to plate boundary activities. As that the Hawaiian magma source is generated discussed in Chapter 1 , hotspots tap magmas from a mantle enriched from the recycling of from different depths such as the lower crust, eclogite (subducted ocean lithosphere), which upper mantle or mantle– core boundary. magmatically mixes with mantle peridotite. Geochemical analyses of basalt and seismic Using geochemical constraints such as high tomography continue to provide insight into NiO concentrations, Sobolev et al. (2005) our attempts to understand these perplexing estimate that recycled (subducted) ocean crust igneous processes. Our discussion of oceanic accounts for up to 30% of the Hawaiian intraplate magmatism has centered on varie- source material. These important studies ties of basalt, which dominate these settings. suggest that the Hawaiian intraplate magma- In sharp contrast, continental intraplate tism may be related to ancient subduction magmatism produces a wider range of 286 EARTH MATERIALS igneous rock types, discussed in the follow- basalts erupted between 191 and 205 Ma, ing section. with a peak age of 200 Ma. CAMP rocks consist of tholeiitic to andesitic basalts, with rare alkaline and silicic rocks. CAMP tholei- 10.4.2 Continental i ntraplate m agmatism ites have low TiO 2 concentrations, negative Continental intraplate magmatism and vol- mantle normalized nobium anomalies and canism produce: moderate to strongly enriched rare Earth element patterns. These geochemical patterns • Continental fl ood basalts. indicate an anomalously hot mantle plume • Continental rift assemblages. that resulted in the partial melting of the over- • Bimodal volcanism. lying lithosphere (Marzoli et al., 1999 ). • Layered basic and ultrabasic intrusions. • Ultrabasic suites that include komatiites Siberian fl ood b asalts and kimberlites. The Siberian fl ood basalts (see Figure 10.11 ) • An unusual array of alkaline rocks and consist predominantly of tholeiitic basalt anorogenic granites. fl ows tens to a few hundreds of meters thick with minor trachyandesites, nephelinites, pic- Continental fl ood b asalts rites, volcanic agglomerates and tuffs (Zolo- tukhin and Al ’ Mukhamedov, 1988 ; Fedorenko Examples of huge outpourings of continental et al., 1996 ). The 251 Ma Siberian fl ood fl ood basalts (CFB) include the Deccan traps basalts were already recognized as one of the of India, Karroo basalts of Africa, Siberian greatest known outpourings of lava when, in fl ood basalts of Russia and the Columbia 2002, the western Siberian Basin fl ood basalt River, Snake River plain and Keweenaw fl ood province was discovered which effectively basalts of the United States. The three largest doubled the aerial extent of the Siberian traps fl ood basalt events – the Permo- Triassic to approximately 3,900,000 km 2 (Reichow et Siberian traps, the Triassic – Early Jurassic al., 2002 ). It is analogous to burying half of Central Atlantic Magmatic Province and the the contiguous United States in lava. In the Cretaceous– Tertiary Deccan traps – corre- Maymecha- Kotuy region of Russia, Kamo et spond with the largest extinction events in al. (2003) suggest that the entire 6.5 km thick Earth ’ s history (Renne, 2002 ). basalt sequence erupted within ∼ 1 million Although less common, silicic large igneous years, based upon U/Pb dates obtained provinces (SLIP) also occur in association from the base (251.7 ± 0.4 Ma) and top with continental break- up, intraplate magma- (251.1 ± 0.3 Ma) of the basalt sequence – tism and backarc basin magmatism. SLIP are truly mind boggling in scale. silicic - dominated provinces containing rhyo- lite caldera complexes and ignimbrites. SLIP Deccan t raps occur notably in the Whitsunday volcanic Over 1,000,000 km 3 of fl ood basalt erupted province of eastern Australia, the Chon Aike in southwestern India between 65 and 69 Ma Province of South America and Yellowstone (Courtillot et al., 1988 ). The Deccan traps (Bryan et al., 2002 ). Below we briefl y describe (see Figure 10.11 ) encompass an area of several well- known continental fl ood basalt 500,000 km 2 in western India. Individual lava provinces beginning with the CAMP. fl ows generally vary from 10 to 50 m in thick- ness with total fl ow thicknesses varying from Central Atlantic Magmatic Province ( CAMP ) less than 100 m to more than 2 km (Ghose, The CAMP formed during the Early Jurassic 1976 ; Sano et al., 2001 ). The fl ood basalts break - up of the Pangea supercontinent, which and related dike swarms are interpreted to produced rift basins and fl ood basalts in result from rifting as the Indian Plate migrated North America, South America, Europe and over a mantle plume (Muller et al., 1993 ). Africa (see Figure 10.11 ). These once contigu- The Deccan traps are dominated by tholeiitic ous tholeiitic basalts are now widely dispersed basalts with minor amounts of alkalic basalts. across the Atlantic Ocean realm, encompass- Geochemical studies suggest that the Deccan ing a total area of more than 7 million km2 . basalts originated by fractional crystallization The Ar 40 /Ar 39 ages indicate that the CAMP of shallow magma chambers ( ∼ 100 kPa, IGNEOUS ROCK ASSOCIATIONS 287

1150 – 1170 ° C). The basaltic magma experi- the high SiO 2 concentrations, high total FeO enced variable degrees of contamination as it (9.5 – 17.5 wt %) and low MgO concentra- ascended and assimilated granitic crustal tions (3– 8 wt %) suggest that the parental rocks (Mahoney et al., 2000 ; Sano et al., magma was not primary (Hooper, 1982 ; 2001 ). Lange, 2002 ). A second set of hypotheses assert that the basalts are the product of Columbia River fl ood b asalts diversifi cation processes (McDougall, 1976 ; Although relatively small compared to the Reidel, 1983 ). Viable diversifi cation models fl ood basalt provinces listed above, the suggest that partial melting of pyroxenite Columbia River fl ood basalts (see Figure (Reidell, 1983 ) or eclogite (Takahashi et al., 10.11 ) are among the most studied CFB on 1998 ) parental rock was followed by the Earth. The Columbia River fl ows consist injection of separate magmatic pulses, subse- largely of quartz tholeiites and basaltic andes- quent magma mixing and assimilation of ite, with 47 – 56 wt % silica (Swanson and crustal rock. Trace element data suggest that Wright, 1980 ; Reidell, 1983 ). Columbia River the Columbia River basalts were not derived basalts crop out in the US states of Washing- by fractionation of a single magmatic pulse. ton, Oregon and Idaho, encompassing an area Thus, a unifi ed model suggests that the of approximately 163,700 ± 5000 km 2 (Tolan Columbia River basalts were created by mul- et al., 1989 ). The total volume of erupted lava tiple pulses of heterogeneous mantle - derived has been estimated to be approximately magmas, contaminated by continental crust 175,000 ± 31,000 km 3 (Tolan et al., 1989 ). during magma ascent and magma mixing The Columbia River basalt group has been (Hooper, 1982 ; Riedel, 1983 ). The tectonic subdivided into fi ve formations: the Imnaha origin of the Columbia intraplate magmatism Basalt, Grande Ronde Basalt, and coeval has been the subject of debate. Possible tec- Picture Gorge, Wanapum Basalt and Saddle tonic causes include: heating following sub- Mountain Basalt. The Grande Ronde Basalt, duction of the Juan de Fuca ridge, backarc which erupted 15.5 – 17 Ma, comprises spreading, the Yellowstone hotspot and con- approximately 87% of the total volume of the tinental rifting. Columbia River basalt (Swanson and Wright, 1981 ). Over 300 individual lava fl ows erupted Continental r ifts from northwest trending fractures between 6 and 17 Ma, making this the youngest conti- Continental rifts produce a wide array of nental fl ood basalt province on Earth. Indi- rocks that include alkalic basalt as well vidual fl ows traveled as much as 550 km, as alkaline and silicic rocks. Alkaline rocks erupting in north– central Idaho and fl owing include phonolite, trachyte and lamproite. to the Pacifi c Ocean (Hooper, 1982 ). Unlike Silicic rocks include rhyolite and rhyodacite, most other fl ood basalt provinces, the Colum- which occur in lava domes or as pyroclastic bia River basalts lack early picritic basalt fl ow and ash fall deposits. Plutonic rocks vary eruptions and interbedded silicic lavas and from syenite and alkali granite to gabbroic have less than 5% phenocrysts (Durand and rocks. Sen, 2004 ). The low concentrations of phen- Continental rifting occurs in regions such as ocrysts are thought to be related to either the East African rift basin, Lake Baikal rapid ascent of magma (McDougall, 1976 ) or (Russia), the Basin and Range and the Rio to a high water content of ∼ 4.4%, which Grande rift system (USA). The East African effectively lowered the melting temperature rift system (Figure 10.13 ) erupts abundant and inhibited the early development of large alkali basalt as well as phonolite, trachyte, crystals (Lange, 2002 ). rhyolite and carbonatite lava. Ancient conti- Various hypotheses have been proposed for nental rifts include the Permian age ( ∼ 250 Ma) the origin of the Columbia River basalt fl ows. Rhine Graben (Germany) and Triassic One set of hypotheses propose that the basalts ( ∼ 200 Ma) Oslo Graben (Norway) and rift crystallized from primary magma (Swanson basins of the Atlantic Ocean basin and the and Wright, 1980, 1981). The relatively low 1.1 Ga Keweenaw rift of the Lake Superior Sr 87 /Sr86 (0.7043– 0.7049) ratios indicate a basin (USA). Continental rift zones can contain mantle source (McDougall, 1976 ). However, important hydrocarbon reservoirs because of 288 EARTH MATERIALS

EURASIAN PLATE Persian Gulf ARABIAN

RED PLATE Nile AFRICA River SEA AFRICAN PLATE (Nubian) 'Erta 'Ale ATLANTIC Gulf of Aden OCEAN INDIAN PLATE

Equator Oldoinyo Lengai AFRICAN Lake PLATE Victoria (Somalian)

Plate boundaries East African rift zone

Figure 10.13 The East African rift system represents the third leg to the Gulf of Aden and Red Sea rift chain. (Courtesy of the US Geological Survey.)

the rapid deposition of organic- rich sediments. All of these forces have the potential to Volcanic fl ows and associated shallow intru- generate continental rifts. sives can also provide valuable metallic ore deposits such as nickel and copper and plati- Bimodal v olcanism num group elements. What is the driving force behind the litho- The widespread occurrence of basalt and rhy- spheric extension that leads to the develop- olite without signifi cant andesite is referred to ment of continental rifts? Various hypotheses as bimodal volcanism (Section 8.4 ). Bimodal have been proposed, which include (Figure volcanism occurs at continental rifts and 10.14 ): hotspots underlying continental lithosphere. Partial melting of the mantle generates basal- • Upwelling of hot plumes generated by tic magma. The rising basaltic magma par- the return convective loop of downgoing tially melts continental crust, resulting in oceanic lithosphere. the dual occurrence of basalt and rhyolite. • Partial melting at great depths of over- A classic example occurs in Yellowstone thickened continental lithosphere follow- National Park in Wyoming (USA). Yellow- ing supercontinent assembly. stone’ s magmatic source is related to a mantle • Subduction of ocean spreading ridges hotspot that has been active for at least 17 resulting in shallow sub- lithospheric million years. The Yellowstone hotspot may melting producing backarc basin type have provided the source material for the extension within the continental immense Columbia River fl ood basalts in lithosphere. Idaho, Oregon and Washington as well as the IGNEOUS ROCK ASSOCIATIONS 289

Lithosphere

Partial melting at Ocean base of spreading overthickened ridge T1 continental lithosphere migrates Partial melting towards of subducted slab trench in T2 and ascending Time 1 Shallow subduction of convective loop ocean spreading ridge in Time 2

Figure 10.14 Possible tectonic causes for continental rifts.

northern parts of California and Nevada. Most of the magma producing the Columbia River fl ood basalts erupted 15– 17 million years ago. Since that time, as North America has migrated in a southwest direction, the position of the active hotspot has migrated ∼ 800 km in a northeasterly direction to its present location at Yellowstone. Christiansen (2001) recognized three immense rhyolitic lava deposits at Yellow- stone ’ s silicic large igneous province: the 2.1 Ma Huckleberry Ridge Tuff, the 1.3 Ma Mesa Falls Tuff and the 640,000- year - old Lava Creek Tuff. Together, these three tuff deposits constitute the Yellowstone Group. The Huckleberry Ridge eruption dispersed Figure 10.15 Dual columnar basalt fl ows are 2450 km 3 rhyolite deposits over an area of separated above and below by massive 15,500 km 2 and produced a caldera over rhyolite ignimbrite deposits in the Yellowstone 75 km long. The Mesa Falls eruption Caldera, USA. (Photo by Kevin Hefferan.) produced tuff deposits largely within the Huckleberry Ridge Caldera. While the Mesa Falls eruptive deposits were restricted to the pre - existing caldera, a new 16 km caldera basalt eruptions appear to be independent of developed along the northwest end of the the rhyolite eruptive cycles. The rhyolite and Huckleberry Ridge Caldera. The youngest basalt eruptions represent two distinctly dif- Lava Creek cycle of eruptive activity began ferent magmatic sources. The rhyolitic magma around 1.2 Ma and continued for approxi- is derived from the successive emplacement of mately 600,000 years. The Lava Creek granitic batholiths within the crust. The basal- eruption produced a large caldera and tic magma is generated by partial melting of scattered rhyolitic deposits over an area of the peridotite- rich upper mantle. The Yellow- 7500 km 2 . stone Caldera consists of two ring fracture Thus the Yellowstone Caldera is a compos- zones within this composite caldera structure. ite caldera generated by three separate rhy- Ring fractures are circular fracture sets olitic eruptive events. In the intervening time generated by ground subsidence following between each of these rhyolitic eruptions, the release of magma from a shallow pluton basaltic lava also erupted (Figure 10.15 ). The (Chapter 8 ). 290 EARTH MATERIALS

Approximately 40 rhyolite eruptions have occurred in the past 640,000 years, since the last of the three cataclysmic Quaternary eruptions at Yellowstone. No lava has erupted in Yellowstone over the past 70,000 years. Two resurgent domes are currently being constructed within the Yellowstone Caldera and the ground surface is slowly being infl ated, with uplift as much as 1 m since the 1920s. While the eruption of lava at Yellowstone is not anticipated in the next few thousand years, the area is presently experiencing uplift, perhaps the early warning signs of a new eruptive phase (Christiansen, Figure 10.16 Close up of rhythmic layers 2001 ). Yellowstone has been the subject of within a channel structure in the Stillwater a movie entitled Supervolcano , which is Complex. (Photo by Kevin Hefferan.) entirely appropriate: eruptions there were among the largest on Earth. The magma that erupted from Yellowstone 2.1 million years ago was approximately 6000 times greater tions, magma mixing or chemical diffusion than the volume released in the 1980 erup- can produce discrete layering in complex tion of Mt St Helens. The smallest of Yel- intrusive bodies. Layers generated by these lowstone ’ s three Quaternary eruptive events processes occur on the scale of meters, released fi ve times more debris than the centimeters or as microscopic cryptic lenses. massive 1815 Tambora (Indonesia) eruption. Layers may occur as fl at, planar structures or It is estimated that 25,000 km 3 of magma are display features commonly associated with contained within the 7 km deep Yellowstone sedimentation such as cross - bedding, graded batholith. Should a portion of that magma bedding, channeling (Figure 10.16 ) or slump erupt from the Yellowstone Caldera, North structures. Cryptic (hidden) layering is America would experience a devastating revealed only by subtle changes in chemical eruption unlike any other witnessed in composition. human history. As with Alaskan- type intrusions, layered basic– ultrabasc intrusions are highly valued Layered b asic – u ltrabasic i ntrusions for metal deposits, particularly platinum group elements (PGE) as well as chromium, Layered basic – ultrabasic intrusions are ano- nickel and cobalt. Metallic ore enrichment is rogenic bodies injected into stable continental likely due to a combination of factors that cratons at moderate depths. Layered intru- include original high concentrations of chro- sions include shallow tabular sills and dikes mium, nickel, cobalt and PGE in magnesium - as well as funnel- shaped lopoliths. These rich, refractory magmas as well as subsequent intrusions commonly contain layers of rocks remobilization and concentration by halogen- such as norite, gabbro, anorthosite, pyroxen- rich (e.g., chlorine) fl uids derived from the ite, dunite, troctolite, harzburgite and lherzo- assimilation of crustal rock (Boudreau et al., lite. Minor silicic rocks such as granite can 1997 ). also occur. Common major minerals include Three of the largest layered intrusions on olivine, orthopyroxene (enstatite, bronzite, Earth are the Stillwater Complex in Montana, hypersthene), clinopyroxene (augite, ferroau- the Bushveld Complex in South Africa and gite, pigeonite) and plagioclase. the Skaergaard Intrusion in Greenland. Other Layered intrusions develop by differentia- signifi cant layered intrusions include the tion of eclogite– peridotite parent magmas Muskox Intrusion of the Northwest Territo- resulting in mineral segregation within a ries (Canada), the Keweenaw and Duluth pluton. In addition to closed- system differen- Intrusion of Minnesota (USA) and the Great tiation processes, open - system diversifi cation Dike of Zimbabwe. The 1.1 Ga Duluth processes (Chapter 8 ) such as multiple injec- Complex , formed during the Keweenaw rift IGNEOUS ROCK ASSOCIATIONS 291 event, is a major undeveloped PGE source. Cawthorn, 1999 ). These include, from top to Plans are currently underway to begin mining bottom, the following: PGE in the Duluth Complex within the next few years. 1 Upper zone consisting of gabbro and norite. 2 Main zone containing gabbro and Stillwater Complex anorthosite. The 2.7 Ga Stillwater Complex is a large, 3 Critical zone consisting of anorthosite, layered basic– ultrabasic igneous intrusion in norite and pyroxenite. the Beartooth Mountains of southwestern 4 Basal zone consisting of orthopyroxenite, Montana. The Stillwater Complex is exposed harzburgite, dunite and peridotite. A along a northwesterly strike for a distance chromite horizon occurs at the top of the of 48 km, with observable thicknesses up basal series. to 6 km. The Stillwater Complex, which formed when basic magma intruded meta- The Bushveld Complex hosts the largest sedimentary rocks, is the fi nest exposed reserves of vanadium, chromium and plati- layered intrusion in North America and con- num group metals in the world. PGE are con- tains economic deposits of platinum group centrated within what is referred to as the metals as well as chromium, copper and nickel Merensky Reef within the critical zone. Ano- sulfi des (McCallum et al., 1980, 1999; Premo rogenic granitic rocks capping the complex et al., 1990 ). contain tin, fl uorine and molybdenum. The The Stillwater Complex consists of three Bushveld Complex layering formed through main units, which include a lowermost basal differentiation processes accompanied by a zone, an ultramafi c zone and an upper banded series of magmatic injections, resulting in a zone. The basal zone consists of norite, massive laccolith or domal structure. As in the harzburgite and bronzite- rich orthopyroxenite Stillwater Intrusion described above, chlorine- layers. The ultramafi c zone consists of dunite, rich magmatic fl uids are thought to have harzburgite, bronzite- rich orthopyroxenite played a role in concentrating PGE in the and chromite- rich peridotite layers. The basal Bushveld Complex (Boudreau et al., 1986 ). and ultramafi c zones contain copper, chro- mium and nickel sulfi de ore deposits. The Skaergaard Intrusion upper banded zone consists largely of repeti- Whereas most layered ultrabasic– basic intru- tive layers of alternating norite, gabbro, sions are Precambrian in age, Greenland’ s anorthosite and troctolite and is enriched in 55 Ma Skaergaard Intrusion is the youngest of copper, nickel and PGE ore deposits (McCal- the great PGE- enriched intrusions. The Skaer- lum et al., 1980, 1999 ; Todd et al., 1982 ). gaard lopolith intrusion crops out along Chlorine - rich magmatic fl uids played a key Greenland’ s eastern shores and offers excep- role in leaching background metal deposits tionally good exposures of layering formed within the intrusion and concentrating these by differentiation and convective current metals in discrete enriched layers called reefs structures. The Skaergaard Intrusion, with a within the banded zone (Boudreau et al., volume of 500 km 3 , is heralded as the fi nest 1986, 1997 ; Meurer et al., 1999 ). example on Earth of fractional crystallization, displaying layered sequences of euhedral to Bushveld Complex subhedral crystals as well as distinctive struc- South Africa’ s 2.06 Ga Bushveld Complex , a tures usually associated with sedimentary massive laccolith or domal structure, is the beds. These structures include cross - bedding, world ’ s largest layered igneous intrusion. graded bedding and slump structures (Wager Extending over 400 km in length, up to 8 km and Deer, 1939 ; Irvine, 1982 ; Irvine et al., thick and underlying an area of 60,000 km 2 , 1998 ). this complex contains a layered sequence of Zoned and layered ultrabasic– basic basic and ultrabasic rocks, capped locally by intrusive complexes provide rare but granite. massive examples of magma diversifi cation The Bushveld Complex consists of four yielding segregated mineral zones and main zones (Daly, 1928 ; Vermaak, 1976 ; valuable metallic ore deposits. 292 EARTH MATERIALS

(a) (b)

Figure 10.17 Microphotograph (a) and fi eld photograph (b) of spinifex texture komattites. (Photos courtesy of Maarten de Wit.)

Other u ltrabasic s uites: i ntraplate v olcanics and Nesbitt, 1982 ). In addition to spinifex texture, s hallow i ntrusives circular varioles, radiating spherulites and tree- like dendritic textures also occur. These Komatiites textures are attributed to rapid undercooling Komatiites are ultrabasic volcanic rocks found (Chapter 8 ) or quenching of extremely hot almost exclusively in Archean ( > 2.5 Ga) lavas (Fowler et al., 2002 ). greenstone belts. Greenstone belts are meta- Nearly all komatiites erupted during the morphosed assemblages of green - colored Archean Eon when the early Earth was much rocks that contain layers of ultrabasic and hotter. Komatiites indicate elevated liquidus basic rocks overlain by silicic rocks and sedi- temperatures of 1575– 1800 ° C (1 atmosphere ments (Chapter 18 ). Komatiites, named after pressure) in the Archean upper mantle (Green the 3.5 Ga Komatii region of Barberton, South et al., 1975 ; Arndt, 1976 ; Wei et al., 1990 ; Africa, are high magnesium ( > 18% MgO), Herzberg, 1992, 1993 , in de Wit, 1998 ). The olivine - rich volcanic rocks, depleted in tita- virtual absence of Phanerozoic komatiites nium and LREE. The high magnesium content may be attributed to lower upper mantle tem- and LREE depletion indicate a previously peratures which precludes the extensive depleted mantle source (Sun and Nesbitt, mantle melting required to produce ultraba- 1978 , in Walter, 1998 ). Komatiite fl ows, fi rst sic melts. The only known Phanerozoic recognized in the Barberton region of South ( < 544 Ma) komatiites occur on Gorgona Africa in 1969, commonly contain spinifex Island, Colombia, where 88 Ma komatiites texture (Figure 10.17 ). Spinifex texture con- erupted as > 1500 ° C ultrabasic lava fl ows. sists of needle - like, acicular olivine, pyroxene Gorgona Island, located 80 km west of (augite and/or pigeonite) and chromite phen- Colombia in the Pacifi c Ocean, is composed ocrysts in a glassy groundmass (Viljoen and largely of gabbro and peridotite (Echeverria, Viljoen, 1969 ; Arndt, 1994 ). Spinifex texture 1980 ; Aitken and Echeverria, 1984 ). Gorgona commonly occurs in the upper parts of Island is also notable for the rare occurrence komatiite fl ows or in the chilled margins of of ultrabasic pyroclastic tuffs which record sills and dikes where rapid quenching pro- explosive volcanism (Echeverria and Aitken, duced skeletal, acicular crystals (Arndt and 1986 ). IGNEOUS ROCK ASSOCIATIONS 293

Hypotheses for the origin of komatiites that kimberlite eruptions generate up to include: 10,000 m 3 of pyroclastic debris over hours to months, producing Plinian ash plumes up • Melting in the hydrated mantle wedge to 35 km high (Chapter 9 ). Strangely, no above the subduction zones (Allegre, ultrabasic lavas have been documented with 1982 ; Grove et al., 1997 ; Parman et al., kimberlite deposits. This is probably due to 2001 ). their low preservation potential and the • A deep mantle plume hotspot that led to extremely high volatile (up to 20%) content large degrees of partial melting producing of kimberlite magma, which can produce oceanic plateaus (Storey et al., 1991 ). 70% vesiculation in the erupting lava (Sparks • Partial melting (10 – 30%) of a garnet peri- et al., 2006 ). dotite at pressures of 8 – 10 GPa (Walter, Kimberlite eruptions form maar craters 1998 ). (Chapter 9 ) that largely fi ll with brecciated, pyroclastic debris (Dawson, 1980 ; Mitchell, Komatiites, like layered gabbroic intrusions, 1986 ; Sparks et al., 2006 ). Due to the associa- are associated with valuable metallic ore tion of high temperature, pressure, volatile deposits such as nickel, copper and platinum content and velocity, kimberlites commonly metals. For example, komatiite metallic ore exhibit extensive hydrothermal alteration deposits occur in the 2.7 Ga Yilgarn Craton and are intensely fractured (Dawson, 1980 ). of Western Australia, the 3.5 Ga South African Altered brecciated olivine and phlogopite Barberton region and the 2.7 Ga Canadian phenocrysts occur within a fi ne groundmass Shield. The nickel sulfi de ore deposits are of serpentine, calcite and olivine. Olivine con- thought to have originated in ultrabasic lava stitutes the major mineral in the vast majority tubes (Chapter 9 ) that concentrated high of kimberlites. However, in many samples density metals in channel beds. olivine is completely replaced by serpentine, mica or clay minerals (Skinner, 1989 ). Kim- Kimberlites berlites also contain the high pressure miner- Kimberlites are brecciated, magnesium- rich, als pyrope garnet, jadeite pyroxene and ultrabasic rocks that rapidly rise to Earth ’ s diamond, which are stable at mantle depths surface via cylindrical diatremes (Chapter 8 ) > 150 km. from deep within the mantle. Diatremes vary Kimberlites were fi rst discovered in the greatly in surface area, ranging from a few Kimberly region of South Africa where they square meters to square kilometers. Most dia- are intimately associated with diamonds. tremes taper downward, resembling an Although best known from South Africa, inverted cone in cross - section view. Kimberl- kimberlites crop out in continental lithos- ite pipes occur with other plutonic structures phere throughout the world, commonly such as dikes and sills. Kimberlites, which occurring with carbonatites and alkaline originate at temperatures of 1200 – 1400 ° C igneous rocks. Carbonatites – igneous rocks and depths exceeding 150 km, rise explosively enriched in carbonate minerals such as calcite, through thick continental lithosphere. dolomite or ankerite – are important CO2 Volatile, enriched, very low viscosity energy sources propelling kimberlites up from mantle melts rocket upward towards Earth ’ s mantle depths. Kimberlites are also associated surface at velocities of ∼ 15 – 72 km/h (Sparks with reactivated shear zones and fracture et al., 2006 ). The magma is propelled upward zones (White et al., 1995 ; Vearncombe and by either the degassing of CO 2 - enriched Vearncombe, 2002 ). Kimberlites occur pri- magma or by phreatomagmatic processes marily in Early Proterozoic to Archean age (Chapter 9 ). Phreatomagmatic processes cratons (2 – 4 Ga), although kimberlites as require a water source to interact with the young as Tertiary age ( ∼ 50 Ma) are known kimberlite magma. The high volatile content (Dawson, 1980 ). serves two primary purposes in that (1) it lowers the melting temperature preventing Carbonatites, l amprophyre, l amproites and crystallization, and (2) it provides the propel- a norogenic g ranites lant “ jet fuel ” to accelerate kimberlite magma In addition to kimberlites, other rare to Earth’ s surface. Sparks et al. (2006) suggest and unusual rocks that occur in continental 294 EARTH MATERIALS lithosphere include carbonatites, lampro- region. Signifi cant volumes of anorogenic phyres and lamproites. These SiO 2 - undersat- granites occur in Precambrian cratons urated rocks typically occur in shallow throughout the world. These mid - Proterozoic (hypabyssal), volatile- rich dikes and may be granitoid rocks are remarkably similar in associated with kimberlites. age, composition and appearance, displaying Carbonatites are shallow intrusive to vol- rapakivi texture. Rapakivi texture refers canic rocks that contain > 20% CO 3 minerals to sodium plagioclase overgrowths on pre - such as natrolite, trona, sodic calcite, magne- existing orthoclase crystals. site and ankerite as well as other minerals A number of models have been proposed such as barite and fl uorite. The origin of for the origin of A- type granites. One model carbonatite was a contentious issue prior to proposes the overthickening of continental the 1960 eruption of the Oldoinyo L ’ Engai lithosphere such that the upper mantle and Volcano in Tanzania. Oldoinyo L’ Engai base of the crust partially melt generating erupted unusually low viscosity pahoehoe silicic magma that subsequently rises and carbonatite lava at temperatures of ∼ 500 ° C. cools at shallower depths to form anorogenic Carbonatites form in stocks, dikes and cylin- granite. Other “ residual source models” drical structures primarily at continental rifts propose that A- type granites, such as Pikes (Dawson, 1962 ). Peak Batholith in Colorado (USA), develop Lamprophyres are magnesium- rich, vola- from the partial melting of residual silicic tile - rich, porphyritic rocks containing mafi c granulite rocks (Chapter 18 ) that had previ- phenocrysts such as biotite, phlogopite, ously generated I - type granites (Barker et al., amphibole, clinopyroxene and melilite. Lam- 1975 ; Collins et al., 1982 ). Alternative models prophyres are associated with kimberlites and suggest that A - type granites are derived by continental rift zones, but also occur as dikes melting quartz diorite, tonalite or granodior- intruding granodiorite plutons at convergent ite parent rocks (Anderson, 1983 ). margin settings. In Chapters 7 – 10 we have presented a Lamproites are potassium - rich, peralkaline logical approach to the description, classifi ca- rocks containing minerals such as leucite, tion and origin of igneous rocks and land- sanidine, phlogopite, richterite, diopside and forms. We have also demonstrated the tectonic olivine. Lamproites are enriched in barium relations in an understandable framework. ( > 5000 ppm), lanthanum (> 200 ppm) and zir- Hopefully we have been somewhat successful conium ( > 500 ppm). In contrast to lampro- in helping you understand igneous processes. phyres and carbonatites, lamproites are The scope of this text requires us to limit our relatively poor in CO 2 ( < 0.5 wt %). Lam- discussion of important topics. For more proites occur in areas of thickened lithosphere detailed discussions beyond the scope of this that have experienced earlier plate conver- textbook, the reader is referred to excellent gence or rifting episodes. petrology textbooks by Winter (2009) , Anorogenic (A- type) granites are silicic Raymond (2007) , McBirney (2007) , Best plutonic rocks that are not associated with (2003) , Blatt and Tracy (1996) , Philpotts convergent margin tectonism. A - type granite (1990) , Ragland (1989) and Hyndman (1985) environments include stable cratons, conti- among others. nental rifts, ocean islands and inactive, post - In succeeding chapters, we will investigate collisional continental margins. Anorogenic how igneous rocks are altered in two ways: granite, alkali granite and syenite were par- (1) by weathering and erosion at Earth ’ s ticularly common 1.1 – 1.4 Ga following the surface, and (2) through the effects of high assembly of the mid - Proterozoic Columbia temperatures, pressures and hot fl uids via Supercontinent. These A - type, granitic intru- metamorphic reactions. In all of these reac- sions are widespread in North America, tions, water plays a critical role in altering extending from Mexico to the Lake Superior and mobilizing elements within Earth ’ s crust.