Metamorphism At Convergent Margins

As oceanic basalt is subducted at convergent margins, it undergoes a series of chemical reactions that both release water into the upper mantle wedge (Section 9.8) and increase the density of the subducting slab. These reactions involve specific metamorphic transformations that reflect the abnormally low geothermal gradients (10°C km-1) and the high pressures associated with the subduction zone environment (Section 9.5).

Prior to its subduction, oceanic basalt may exhibit low pressure (<0.6 GPa)/low temperature (<350°C) metamorphic mineral assemblages of the zeolite and prehnite-pumpellyite facies (Fig. 9.26). In some places greenschist facies minerals also may be present. In basalt, this latter facies typically includes chlorite, epidote and actinolite, which impart a greenish color to the rock (see also Section 11.3.2). This type of alteration of basalt results from the circulation of hot seawater in hydrothermal systems that develop near ocean ridges (Section 6.5).

As the altered basalt descends into a subduction zone, it passes through the pressure-temperature field of the blueschist facies (Fig. 9.26), which is characterized by the presence of the pressure-sensitive minerals glaucophane (a sodic blue amphibole) and jadeite (a pyroxene). A transitional zone, characterized by the presence of lawsonite, also may occur prior to the transformation to blueschist facies. Lawsonite is produced at temperatures below 400°C and at pressures of 0.30.6 GPa (Winter, 2001), conditions that are not yet high enough to produce glaucophane and jadeite. Lawson-ite, along with glaucophane and other amphibole minerals, is an important host for water in subducting ocean crust.

One of the most important metamorphic reactions resulting in the dehydration and densification of subducting oceanic crust involves the transformation from the blueschist facies to the eclogite facies (Fig. 9.26). Eclogite is a dense, dry rock consisting mostly of garnet and omphacite (i.e. a variety of clinopyroxene rich in sodium and calcium). The exact depth at which eclogite facies reactions occur depends upon the pressures and temperatures in the subducting oceanic crust (Peacock, 2003). In relatively cool subduction zones, such as in northeast Japan (Plate 9.3a between pp. 244 and 245), the transformation may occur at depths of >100 km (Fig. 9.31). In relatively warm subduction zones, such as in southwest Japan (Plate 9.3b between pp. 244 and 245), the transformation may occur at depths as shallow as 50 km (Fig. 9.27). This transformation to eclogite enhances the negative buoyancy of the descending lithosphere and contributes to the slab-pull force acting on the subducting plate (Section 12.6).

Metamorphic Facies Kyanite Andalusite

Temperature (0C)

Figure 9.26 Pressure-temperature diagram showing the approximate limits between the metamorphic facies (from Winter, John D, Introduction to Igneous and Metamorphic Petrology, 1st edition © 2001,p. 195. Reprinted by permission of Pearson Education Inc., Upper Saddle River, NJ). Example of an elevated (30°C km~') continental geotherm and stability ranges of three Al2SiO5 polymorphs commonly found in metamorphosed sedimentary rock (Ky, kyanite; And, andalusite; Sil, sillimanite) are shown for reference. Ab and Ep are albite and epidote, respectively.

Temperature (0C)

Figure 9.26 Pressure-temperature diagram showing the approximate limits between the metamorphic facies (from Winter, John D, Introduction to Igneous and Metamorphic Petrology, 1st edition © 2001,p. 195. Reprinted by permission of Pearson Education Inc., Upper Saddle River, NJ). Example of an elevated (30°C km~') continental geotherm and stability ranges of three Al2SiO5 polymorphs commonly found in metamorphosed sedimentary rock (Ky, kyanite; And, andalusite; Sil, sillimanite) are shown for reference. Ab and Ep are albite and epidote, respectively.

Melting Wet Basalt

Figure 9.27 Calculated pressure-temperature paths for the top and base of subducted oceanic crust beneath northeast and southwest Japan (after Peacock & Wang, 1999, with permission from Science 286,937-9. Copyright by the AAAS, © 1999). Metamorphic facies and partial melting curves (dark gray lines) for basalt under wet and dry conditions are shown.

Figure 9.27 Calculated pressure-temperature paths for the top and base of subducted oceanic crust beneath northeast and southwest Japan (after Peacock & Wang, 1999, with permission from Science 286,937-9. Copyright by the AAAS, © 1999). Metamorphic facies and partial melting curves (dark gray lines) for basalt under wet and dry conditions are shown.

Samples of blueschist and eclogite obtained from convergent margins (Section 9.7) provide important information on the physical and chemical conditions that occur within and above subducting lithosphere. Some of the first direct evidence of the conditions in the vicinity of the subduction zone décollement beneath a forearc has been provided by observations in the Mariana forearc. In this setting, large serpentine mud volcanoes up to 30 km in diameter and 2 km high occur in the forearc slope above an erosive margin (Fig. 9.19). In addition to serpentine, the volcanoes erupt slab-derived fluids and blueschist facies clasts that record the relatively cool temperatures of 150-250°C and pressures of 0.5-0.6 GPa (Maekawa et al., 1993). These determinations are consistent with thermal models of the slab-mantle interface where abnormally low geo-thermal gradients result from the rapid descent of cool oceanic lithosphere at trenches (Section 9.5) and from low to moderate levels of friction (Peacock, 1992). Samples of material obtained by drilling the Mariana mud volcanoes also provide evidence of the interactions among pore fluids, sediment and metamorphic rock that occur in an accretionary prism (Fryer et al., 1999). Similar material, known as sedimentary serpenti-nite, occurs in blueschist facies metamorphic belts preserved within continental crust. These belts commonly are interpreted to represent the suturing of ancient continental margins following the consumption of an intervening ocean (Sections 10.4.2, 11.4.3). Blueschist also is associated with ophiolitic suites (Ernst, 1973), lending support to the interpretation that some ophiolites formed in the forearc region of incipient subduction zones (Section 2.5).

In addition to the low temperature/high pressure type of metamorphism associated with subduction zones, some convergent margins also exhibit a type of regional metamorphism characterized by high temperatures (>500°C) and low to moderate pressures. This type of metamorphism commonly is associated with the high geothermal gradients that characterize magmatic arcs. Index minerals in metamorphosed sedimentary rocks, such as andalusite and sillimanite (Fig. 9.26), provide evidence of high temperatures in these regions. Temperature gradients of more than 25°C km-1 up to about 50°C km-1 result from the ascent of magmas generated where aqueous fluids from the subducted slab infiltrate the mantle wedge (Section 9.8). This type of metamorphism also is associated with the high differential stresses, deformation, and crustal thickening that accompany the formation of Andean-type orogens (Section 10.2.5). Both associations affect large areas of the crust at convergent margins and thus reflect the large-scale thermal and tectonic disturbance associated with subduction and orogeny.

The most common groups of rocks associated with regional metamorphism belong to the greenschist, amphibolite and the granulite facies (Fig. 9.26). The transition from greenschist to amphibolite facies, like all metamorphic reactions, is dependent on the initial composition of the crust as well as the ambient pressure, temperature and fluid conditions. In metamorphosed basalt this transition may be marked by the change from actinolite to hornblende as amphibole is able to accept increasing amounts of aluminum and alkalis at high temperatures (>500°C) (Winter, 2001). At temperatures greater than 650°C amphibolite transforms into granulite. Granulites are highly diverse and may be of a low, medium or high pressure variety (Harley, 1989). In general, granulite facies rocks are characterized by the presence of anhydrous mineral assemblages such as orthopyroxene, clinopyroxene, and plagioclase.

If conditions at high temperatures are hydrous, then migmatite may form. Migmatite is a textural term that describes a mixed rock composed of both metamorphic and apparently igneous material. Proposed mechanisms for migmatite formation have included the partial melting of a rock, the injection of igneous (granitic or tonalitic) material into a rock, and the segregation of silicate material from a host during metamorphic rather than igneous activity. Migmatites are best developed in pelitic metasedimentary rocks, but also may occur in mafic rocks and granitoids. Brown et al. (1999) describe the structural and petrologic characteristics of migmatite derived from pelitic and basaltic rocks. Suda (2004) summarizes the formation and significance of migma-tite in an intra-oceanic island arc setting. Klepeis et al. (2003) and Clarke et al. (2005) provide summaries of the tectonic setting and possible interpretations of a high-pressure (1.2-1.4 GPa) mafic granulite and associated migmatite belt located in Fiordland, New Zealand. These latter rocks represent the hot, lower crustal root of thick Cretaceous continental arc crust that has been exhumed during subsequent tectonic activity.

Attempts to place the evolution of the high pressure/low temperature and the high temperature/low pressure varieties of metamorphic rocks in the context of subduction zone processes are common in the scientific literature. One important early effort by Miyashiro (1961, 1972, 1973) led to the concept of

Paired Metamorphic Belts

Figure 9.28 Three paired metamorphic belts in Japan, F-F' is the Itoigawa-Shizuoka Line (from Miyashiro, 1972. Copyright 1972 by American Journal of Science. Reproduced with permission of American Journal of Science in the format Textbook via Copyright Clearance Center). Profile A-A' is shown in Fig. 9.29.

Figure 9.28 Three paired metamorphic belts in Japan, F-F' is the Itoigawa-Shizuoka Line (from Miyashiro, 1972. Copyright 1972 by American Journal of Science. Reproduced with permission of American Journal of Science in the format Textbook via Copyright Clearance Center). Profile A-A' is shown in Fig. 9.29.

paired metamorphic belts. On the Japanese islands of Hokkaido, Honshu, and Shikoku (Fig. 9.28), Miyashiro identified three pairs of metamorphic belts of different age that approximately parallel the trend of the modern Japanese subduction zone. Each of these belts consists of an outer zone of high pressure/low temperature blueschist and an inner belt of low pressure/ high temperature rock. This spatial relationship and the similar age of each outer and inner belt led him to conclude that the belts formed together as a pair. After the introduction of plate tectonics, these paired belts were interpreted to be the result of underthrust-ing of oceanic crust beneath an island arc or continental crust (Uyeda & Miyashiro, 1974). The outer metamorphic belt was interpreted to develop near the trench due to the low geothermal gradient caused by subduction. The inner belt was interpreted to form in the arc, some 100-250 km away, where geothermal gradients are high.

The application of the paired metamorphic belt model to Japan has allowed some investigators to infer the direction of subduction and plate motions at various times in the past. At present, Pacific lithosphere is subducted in a northwesterly direction beneath the Japan arc. The metamorphic polarity of the Sangun/Hida and Ryoke/Sanbagawa paired belts (Fig. 9.28) suggests that they were formed similarly, by underthrusting in a northwesterly direction. The Hidaka/Kamuikotu paired belt shows the opposite metamorphic polarity, and therefore may have formed during a different phase of plate movements when the direction of subduction was from the west of Japan. However, there are some discrepancies in this interpretation. For example, the Ryoke/Sanbagawa belts are much closer together than

Jurassic accretionary prism Sangun belt

Hida belt

Sanbagawa metamorphic belt

Jurassic accretionary prism and Paleozoic rocks

Shimanto accretionary Nankai accretionary prism prism

Jurassic accretionary prism Sangun belt

Hida belt

Jurassic accretionary prism and Paleozoic rocks

Shimanto accretionary Nankai accretionary prism prism

Magmatism Accretionary Prism

20 m

Mantle lithosphere

30 40

Figure 9.29 Geologic cross-section of southwest Japan (modified from Taira, 2001, Annual Review of Earth and Planetary Sciences 29, Copyright © 2001 Annual Reviews). Location of profile shown in Fig. 9.28. MTL, Median Tectonic

Mantle lithosphere

20 m

30 40

Figure 9.29 Geologic cross-section of southwest Japan (modified from Taira, 2001, Annual Review of Earth and Planetary Sciences 29, Copyright © 2001 Annual Reviews). Location of profile shown in Fig. 9.28. MTL, Median Tectonic

predicted by the model, and so it has been suggested that the boundary between them, called the Median Tectonic Line, experienced some 400 km of strike-slip movement (Section 5.3). This transcurrent movement has been confirmed by detailed mapping (Takagi, 1986) and indicates that strike-slip faulting was responsible for bringing the Sanbagawa and Ryoke belts into juxtaposition (Fig. 9.29).

Since the work of Miyashiro (1961, 1972, 1973), interpretations of paired metamorphic belts have been attempted in both island arc and Andean type settings around the Pacific margin (Fig. 9.30). The simplicity of these interpretations is appealing; however, in some examples, numerous inconsistencies exist. In the Atlantic region and in the Alps, many Phanerozoic metamor-phic belts either lack one of the pairs or the contrast between them is unclear. These patterns, and the realization that many paired metamorphic belts did not form in their present positions, has led to skepticism about their overall significance. Brown (1998) summarized the evolution of thought that has led to a general demise of the concept of paired metamorphic belts in many convergent margins and orogens. One reason for this is that most metamorphic belts are no longer considered to be characterized by a single geothermal gradient, mainly because the rocks record an evolution across a range of geotherms through time. In addition, the recognition of suspect terranes and the importance of accretionary processes (Section 10.6) suggests that the tectonic units along these margins reflect a complex array of processes that may or may not have accompanied subduction. Taira (2001) summarizes the importance of terrane collision for the evolution of Japan's metamorphic belts.

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