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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 ).
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