Introduction to Ophiolites
1. Ophiolite and oceanic lithosphere
Ophiolite is a stratified igneous rock complex composed of upper basalt member, middle gabbro member and lower peridotite member (Fig. 1). Some large complexes measure more than 10 km thick, 100 km wide and 500 km long. The term "ophiolite" means "snake stone" in Greek. Basalt and gabbro are commonly altered to patchy green rocks, and peridotite is mostly changed into black, greasy serpentinite. The term comes from such serpentine appearance of these altered, metamorphosed, or sometimes highly fragmented members. Ophiolite is interpreted to be thrust sheet of ancient oceanic lithosphere which has been obducted over the continental crust in the course of orogeny. The ophiolite succession can be correlated with the seismologic layering of the oceanic lithosphere (Fig. 1). The sedimentary cover correspond to Layer 1, basaltic pillow lava matches Layer 2, sheeted dikes and gabbro with occasional plagiogranite intrusions are correlated to Layer 3, and ultramafic cumulates and residual mantle peridotite represent Layer 4 (mantle).
Figure 1. Opholite succession and seismeic layer of oceanic crust
2. Ophiolite examples and their occurrences
Ophiolite was first described from the Alps in the early 20th century, and was later discovered from almost every orogenic belt on the earth. Semail ophiolite in Oman (Mesozoic), Troodos ophiolite in Cyprus (Mesozoic), Papua ophiolite in Papua-New Guinea (Mesozoic), and Bay of Islands ophiolite in Newfoundland (Paleozoic) are the best known. Yakuno (Paleozoic), Horokanai (Mesozoic) and Poroshiri (Mesozoic) are the three full-membered ophiolites in Japan, which also has many dismembered ophiolites such as Oeyama (Paleozoic), Miyamori (Paleozoic), Mikabu (Mesozoic) and Setogawa-Mineoka (Cenozoic).
Ophiolite occurs as a nappe (intact thrust sheet) or as a melange (tectonic mixture of fragments). In collisional orogenic belts, ophiolites generally lie on older continental basement. In circum-Pacific orogenic belts, however, ophiolites generally lie on younger accretionary complexes. For example, Jurassic Tamba accretionary complexes are overlain by the late Paleozoic Yakuno ophiolite, which is in turn overridden by the early Paleozoic Oeyama ophiolite (Fig. 2). The younger Mikabu and Setogawa-Mineoka ophiolites underlies the Jurassic accretionary complexes in the Pacific coastal areas.
Figure 2. Pre Cretaceous tectonic units of the Inner Zone of southwestern Japan
3. Petrologic classification of ophiolites
Ophiolites may have formed either at divergent plate boundaries (mid-oceanic ridges) or convergent plate boundaries (supra-subduction zones; i.e. island arcs and marginal basins). They are called MOR and SSZ types, respectively. These types are identified by chemical composition of the rocks and minerals in comparison with those from various tectonic settings on the earth at present.
Ophiolitic mantle peridotite is the refractory residue after extraction of basaltic melt through partial melting processes in the mantle. Although primary mantle peridotite may be lherzolite with abundant clinopyroxene, it changes into clinopyroxene-poor (or -free) harzburgite as the degree of melting increases (Fig. 3). The mantle peridotite samples dredged from the mid-oceanic ridges are mostly lherzolite, while those dredged from supra-subduction zones (trench walls) are mostly harzburgite.
Figure 3. Modal variation of residual mantle peridotite with increasing degree of melting
Ophiolitic igneous cumulates shows systematic variation in the crystallization sequence of minerals corresponding to the petrologic diversity of the underlying peridotite mantle. The mineral crystallizing next to olivine varies from plagioclase through clinopyroxene to orthopyroxene as the degree of melting in the underlying mantle increases (Fig. 4). The characteristic cumulate rocks correspondingly varies from troctolite through wehrlite to harzburgite.
In general, ophiolitic basalt also varies from alkali basalt or high-alumina basalt (like mid-ocean ridge basalt (MORB)) through low-alumina basalt (like island-arc tholeiite (IAT))to boninite (high-magnesian andesite) in correspondence with the petrologic variation of the underlying members (Fig. 4).
Figure 4. Petrologic variation of the underlying members
4. Ophiolites in the circum-Pacific orogenic belts
Ophiolites in the circum-Pacific orogenic belts generally occur intercalated among the accretionary complexes and show multiple tectonic superposition as exemplified by the Klamath Mountains in western USA (Fig. 5). The oldest early Paleozoic ophiolite occupies structurally uppermost position, and younger ones take lower seats. Such "Confucian" ophiolite belts are also present in Japan and northeastern Russia, and forms "circum-Pacific Phanerozoic multiple ophiolite belts". This structure may be formed by underplating of the accreted oceanic material and trench-fill sediments beneath the overlying SSZ lithosphere (ophiolite) and subsequent underplating of the younger SSZ-trench system. The circum-Pacific ophiolite belts are also characterized by extreme petrologic diversity. Juxtaposition of depleted, clinopyroxene-free harzburgite and fertile lherzolite is common, though such a case is rare in the collisional orogenic belts.
Figure 5. Geologic structure of the Klamath Mountain, western USA (Irwin 1981)
Reported formation ages of ophiolites show three distinct peaks at about 750, 450 and 150 Ma, respectively (Fig. 6). These are called ophiolite pulses. Each pulse corresponds to the period of worldwide magmatic event as represented by voluminous granite intrusions.
Production rate of oceanic crust was distinctly high during the 80 and 120 Ma interval of Cretaceous time, as evidenced by wide area of the ocean floor formed in this interval. Magnetic reversals of the earth, which take place every million years, were unreasonably absent during this interval. These facts lead Larson (1991) to a hypothesis of superplume, a big plume of hot mantle rock which ascended from core/mantle boundary and erupted beneath the South Pacific ocean during this interval, causing worldwide magmatic event. This interval corresponds to the later half of the Mesozoic ophiolite pulse (Fig. 6).
Figure 6. Histogram of formation ages of ophiolites in the world
6. Ophiolite belts on the earth
Ophiolites issued by each pulse tend to form a particular ophiolite belt. Late Proterozoic (ca. 750 Ma) ophiolites are distributed in the Pan-African orogenic belt, early Paleozoic (ca. 450 Ma) ophiolites appear in the Appalachian-Caledonian-Uralian belt, and Mesozoic (ca. 150 Ma) ophiolites dominate the Alpine-Himalayan belt (Fig. 7). However, the circum-Pacific orogenic belts bear ophiolites of widely varying ages, including at least two pulses (early Paleozoic and Mesozoic). This may be due to continuous, subduction-induced, accretionary orogeny that have taken place in the circum-Pacific areas from early Paleozoic to the present, showing contrast to the episodic, short-lived, collisional orogeny in the continental areas. Circum-Pacific ophiolites may be the best witnesses of the history of superplumes.
Figure 7. Ophiolite belts in the world
Author : Akira ISHIWATARI (Dr., Assoc. Prof., Fac. Sci., Kanazawa University)