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The Ophiolite Concept Evolves

The Ophiolite Concept Evolves

PERSPECTIVE THE OPHIOLITE CONCEPT EVOLVES

Robert G. Coleman*

1811-5209/13/0010-082$2.50 DOI: 10.2113/gselements.10.2.82 The lowest part of the mantle section is mostly tectonized ultramafi c rocks consisting mainly of , , , and minor n 1949, little did I realize, as I began my PhD studies chromitite. In most cases these rocks have deformational fabrics that at Stanford University on the mineralogy of the New distinguish them from the overlying layered mafi c and ultramafi c rocks. Idria body in the Diablo Range of central These deformed ultramafi c rocks are generally considered the residue of I partial melting and the source of the overlying layered rocks; however, , that the ideas in my thesis (Coleman 1957) there are instances where they are not related. The transition to the would become anachronistic. At the time I began my fi eld layered sequence is referred to as the Moho discontinuity and is easily work in New Idria, the Johns Manville and Union Carbide located by geophysical means. These layered sequences are complex, with layered mafi c and ultramafi c rocks intruded by later . companies had begun open pit mining for short-fi ber asbestos. Their operations exposed numerous tectonic A sheeted complex characterized by subvertical dikes with igneous textures overlies the plutonic sequence. Extrusive rocks inclusions of blueschist metamorphic rocks containing in ophiolites consist of pillow lavas, pillow , and massive lava jadeite and glaucophane as well as small ophiolitic fl ows, which range in composition from and basaltic andesite at fragments. With these new exposures it became obvious the bottom to andesite, dacite, and rhyolite at the top. Pelagic sedimen- that the New Idria serpentinite massif was not an igneous tary rocks cover these volcanic rocks, and through their , they reveal their travel history within the oceanic from intrusion but instead consisted mostly of highly sheared ridge to trench. mélange within an elongate, diapiric dome protruding Dilek and Furnes emphasize that basal tectonized are into an anticlinal fold in sediments! Today, this depleted mantle residues that have undergone various degrees and famous serpentinite massif is better described as the end episodes of melt extraction from the original primitive mantle. They product of tectonically destroyed ophiolite. conclude that the mineralogy and geochemical compositions of crustal and mantle sequences in most ophiolites do not represent a simple melt–residua relationship. I was invited as a U.S. Geological Survey petrologist to spend a year with the Geological Survey (1961–1962) to investigate Ophiolites are classifi ed to the fi rst order as -related and numerous serpentinite bodies in that country, which are similar in subduction-unrelated types. Those ophiolites whose magmatic con- many ways to those in California. The New Zealand fi eld geologists struction was not affected by subduction processes include continental at this time were actively pursuing 1:250,000-scale mapping of the margin, mid-ocean ridge, and plume-type ophiolites. They display large country, and all rock identifi cation studies were carried out by a small variations on the Th/Yb–Ta/Yb discrimination diagram. Subduction- group of petrologists. Along some of the contacts of the infl uenced ophiolites include suprasubduction zone (SSZ) and volcanic we found fi ne-grained, dense rocks () resembling chilled arc (VA) ophiolites. Specifi c tectonic settings of SSZ oceanic-crust forma- igneous contacts; however, thin section studies revealed the presence tion include the , backarc, and incipient arc. The magmatic and of low- temperature calcium silicates, which were formed from calcium- geochemical evolution of SSZ and VA ophiolites may be controlled by rich, high-pH (11.5) fl uids produced during the serpentinization of the the mode and nature of melting in the mantle and by dehydration of the peridotites. These relationships later reinforced my idea that serpenti- subducting slab and the element fl ux from it into the overlying mantle. nites along continental margins could be tectonically transported and emplaced within zones. The emplacement mechanisms of ophiolites are discussed using simple cross sections with possible geodynamic scenarios. Ophiolites In the decade (1960–1970) following my return to the USGS in Menlo are incorporated into continental margins through complex interac- Park, California, the plate tectonic revolution gained momentum and tions of lithospheric plates. The age, thickness, and thermal state of eager young professors and students of the various campuses of the oceanic lithosphere to be emplaced, the nature and geometry of plate University of California and Stanford produced new interpretations of boundaries involved, and the size and character (e.g. oceanic versus California geology. The term ophiolite had been used to describe ser- continental) of the interacting plates are among the most important pentine associations in Europe and became the descriptor for tectonic factors controlling the dynamics of ophiolite emplacement mecha- fragments of found in California orogenic zones. nisms. Regardless of the nature of the tectonic setting of its magmatic development, subduction zone tectonics is an essential driver for The papers in this issue of Elements are a demonstration of how Earth incorporating preexisting oceanic lithosphere into continental mar- science research has benefi ted from the explosive high-tech revolution gins. Detailed tectonic settings and examples of location are given: in geochemistry and geophysics. Each year brings new publications on , Ligurian (); mid-ocean ridge, fi eld and laboratory studies of ophiolites from all parts of the world, (New Zealand–Southern Ocean); suprasubduction zone, ; volcanic leading to new facts and observations that modify prior interpretations. arc, Magnitogorsk, Urals. A north-polar projection showing the global The article by Dilek and Furnes (2014) includes a notable collection of distribution of Phanerozoic orogenic belts clearly shows that ophiolites current ideas concerning the ophiolite paradigm, which has developed are critical to understanding plate tectonic evolution. It seems that into an essential element of . They describe variations Pandora’s Ophiolite Box has not yet reached the bottom. in the internal structure, generation, geochemical fi ngerprints, Goodenough and others (2014) show that the Oman–United Arab and emplacement tectonics. They also attempt to produce a classifi ca- Emirates (UAE) ophiolite is the largest well-exposed ophiolite on Earth tion that extends beyond the original Penrose ophiolite fi eld conference and perhaps the most studied. It has been divided into 12 separate, defi nition. They speculate on global tectonics and its effects on the fault-bounded blocks; the northern three lie mostly within the UAE. generation and recycling of material in the . The ophiolite in the UAE preserves clear evidence for two stages of magmatism. The earliest episode formed at a spreading center (Phase 1), and a later episode is associated with the onset of subduction (Phase 2). * Dept. of Geological & Environmental Sciences Stanford University Similar two-stage magmatism has been recognized in the Oman sector, Stanford, CA 94305-2115, USA but the UAE contains the most voluminous suprasubduction zone mag-

ELEMENTS 82 APRIL 2014 PERSPECTIVE

matism yet described for this ophiolite. Phase 1 magma formed within a Pearce’s early studies produced a way of fi ngerprinting ophiolites, fi rst mid-ocean ridge spreading center in a marginal basin. Phase 2 hydrous using the immobile elements, namely Ti, Zr, Y, Nb, Cr, and V, and later magmas record the initial stages of SSZ arc magmatism. Phase 1 has a using the elements Th, Ta, Hf, and Sc. New analytical methods (ICP– MORB geochemistry and formed a classic ocean-crust sequence. During MS) have allowed analysis of the full spectrum of immobile elements, Phase 2, irregular bodies intruded into the Phase 1 layered sequence providing a powerful tool for estimating the magmatic and tectonic along thrusts and shear zones. The earliest U–Pb ages of zircons from setting of ophiolites. In this issue, Pearce (2014) carefully presents the Phase 2 SSZ plagiogranites range from 93.5 to 97.9 Ma, while more system he uses to geochemically classify ancient ophiolite magmas. recent dating ranges from 94–96 Ma. The authors have selected 95.3 Ma He describes in detail the use of discriminant diagrams to fi ngerprint as the best representative age for SSZ magmas in the northern UAE magmatic and possible tectonic settings. He fi rst selects the least altered blocks. The authors demonstrate that Phase 2 magmatism is present in magmatic rocks (volcanic glass if possible) from each location and clas- all the northern and central blocks of the Oman–UAE ophiolite, with sifi es them petrologically. Then ICPM–MS analyses are done to identify a general southwards decrease in volume. Phase 2 magmatism is scarce the amount of each immobile element, and the results are plotted on in the Wadi Tayin block, the southeast termination of the ophiolite. discriminant diagrams. He shows that by examining the plotted pat- Early intraoceanic, high-temperature of the ophiolite was terns and comparing them to subduction proxies, one can distinguish marked by metamorphism of rocks at the base of the peridotites and has between mid-ocean ridge and suprasubduction magmas. In addition, been dated at 93–95 Ma. The Saih Hatat folded nappes in the basement using a plume proxy, one can also defi ne the setting of the mid-ocean contain blueschist and metamorphics and structures related ridge ophiolite. to subduction of the Arabian continental margin. Previous authors thought that this high-pressure metamorphism of Pearce emphasizes that immobile element fi n- the nappes was related to the emplacement of the gerprinting of ancient ophiolites is fraught with ophiolite. Older ages for the eclogite (110 Ma) and diffi culties as these rocks may have experienced evidence of its metamorphic retrogression have intense alteration and important tectonic events, led other authors to suggest that subduction of To have been involved in the which may not be recorded in their magmatic history. Arabian and buoyant uplift took plate tectonic revolution was place before ophiolite emplacement. These diverse a learning experience on how To have been involved in the plate tectonic revolu- opinions indicate that a generalized scenario may tion was a learning experience on how to give up not be useful for the entire Oman ophiolite. to give up accepted ideas accepted ideas learned as a student. Earth science Ishizuka and others (2014) provide a compre- learned as a student. research has a long history of evolving. Geologic hensive study of the Izu-Bonin-Mariana (IBM) mapping, satellite imagery, and geophysical forearc, an area measuring 300 km by 3000 km, probes of the crust and mantle now provide a and confi rm its suprasubduction formation as continuous fl ow of new factual data. High-tech oceanic crust developed in the initial stages of an . The fi rst instruments can rapidly reveal quantitative chemical and isotopic rela- volcanic products of the IBM arc are represented by forearc tionships within a single mineral grain. These collected papers illustrate (FAB), consisting mainly of pillow lavas and hyaloclastites. The central how it is now possible to combine some of these new geotech methods Mariana forearc shows a progression from FAB-like lavas at the bottom to better understand large and small Earth systems. No longer is it to nearly boninitic lavas on top, linking these two lava types in time possible for one person to produce all the micro- and megascale data and space. Sheeted dikes in the IBM show that some of the magmatism required to advance the concepts of the ophiolite paradigm. developed in an extensional stress fi eld. These observations indicate My studies of ophiolites have allowed me to carry out fi eld investiga- that the fi rst magmatic activity in the IBM arc was associated with sea- tions in many parts of the world and learn different approaches from fl oor spreading. The peridotites at the lowest stratigraphic level of the other geologists. I organized the fi rst Penrose Conference on ophiol- IBM forearc section are dominated by with veins. ites in 1972, some 180 years after European geologists introduced the Two distinct dunites in equilibrium with two distinct melts produced term. Renewed interest in ophiolite sequences developed during this boninitic and mid-ocean ridge basalt (MORB). FABs from the Bonin time as part of the new plate tectonic paradigm. During the Penrose 40 39 forearc were dated at between 48 and 52 Ma by Ar/ Ar. The Conference, there was not total agreement among the participants, and differentiates ranged from 46 to 48 Ma, and the arc tholeiites who were divided between those who had studied North American ranged from 35 to 47 Ma. ophiolites and those who had studied European ones within the ancient FABs were generated from clinopyroxene-impoverished residual mantle Tethyan ocean. Nevertheless, at the end of the meeting, the participants left after an earlier stage of decompression melting. Most FABs lack were able to produce a consensus defi nition for ophiolite that has been enrichment in large-ion lithophile elements (such as Ba, Sr, Pb) com- useful in providing guidelines for ongoing research. Most ophiolite pared to rare earth elements and high-fi eld-strength elements, implying occurrences are within active or ancient continental margins (sutures). that, like MORB, they have received little or no input of slab-derived The similarity of these allochthonus ophiolite bodies to oceanic crust material. Transitional and boninitic lavas share some geochemical char- stratigraphy was further confi rmed by drilling and seismic imaging of acteristics with the FAB, such as low Ti, low HFSE contents, and very the oceanic crust. low Ti/V ratios. These geochemical features indicate a depleted mantle I introduced the term obduction to describe ophiolite emplacement in source for the magmas of the boninites and transitional lavas. New Caledonia, Papua New Guinea, and Oman associated with older The boninites and related lavas within suprasubduction zone ophiolites blueschist metamorphism. Recent studies in Oman show that the Semail in other worldwide locations are not uniform, and in general MORB-like ophiolite was thrust over older continental margin crust containing basalts predate boninitic rocks. The age distribution, combined with and blueschists, whose isotopic ages are older than the ophio- their geochemical fi ngerprints, suggests that the development of the lite rocks. Cretaceous Tethyan SSZ ophiolites was nearly coeval along the more Recent reports of diamond and other ultrahigh-pressure minerals occur- than 2000 km long belt and that it involved the simultaneous initiation ring as inclusions within Tibetan ophiolitic podiform chromitite with of a subduction zone within the southern Tethys. Recent results from crustal isotope signatures have opened up a completely new aspect the IBM forearc illustrate a similar pattern: FAB erupted fi rst, followed on the evolution of ophiolite mantle rocks within the lithosphere by transitional and then boninitic lavas.

ELEMENTS 83 APRIL 2014 PERSPECTIVE

(Yang et al. 2014). Direct evidence of crustal recycling of ophiolitic peridotites by subduction into the mantle will provide challenging research opportunities for the Earth science community. When did the Earth’s oceans fi rst develop ophiolite-like crust? Indeed it is probable that relict oceanic crust is preserved in granite– greenstone–komatiite terrains or other Archean tectonic settings, but a complete Penrose-type Archean ophiolite has not yet been recognized as evidence for when subduction tectonics began. The presence of ophio- lite mélanges with blueschist metamorphic rocks or UHP metamorphic rocks is necessary evidence. Another unsolved problem concerns the evolution of new oceanic crust within rifting passive margins. During my fi eld mapping with the U.S. Geological Survey in Saudi Arabia, we discovered an igneous complex of sheeted dikes, layered , granophyre, and basaltic–rhyolitic lava on the southeast margin of the Red Sea, which we called the Tihama Asir complex. These rocks were intruded parallel to the present-day axis of Bob Coleman next to massive jadeite veins in blueschist “knocker” in serpentine, California, 2006 the Red Sea and were related to early Tertiary (20–24 Ma) continental extension. Tholeitic magma intruded along faults and fractures in rifted Robert G. Coleman is Professor Emeritus of Geological & continental crust, forming extensive dike swarms and layered gabbroic Environmental Sciences at Stanford University, where he was previ- plutons, along with associated silicic intrusive bodies derived from par- ously a professor (1982–1993). Earlier, he was a geologist at the tial melting of the continental crust. During the Pliocene to Miocene, U.S. Geological Survey in Menlo Park, California (1954–1982). Bob evaporite deposits up to 3 km thick accumulated on the extended Red earned his BS (1948) and MS (1950) degrees in geology at Oregon Sea Basin and are coeval with the Mediterranean salinity crisis. A major State University and his PhD in geology at Stanford University unconformity in the basin at 5 Ma marks the end of salt deposition and (1957). His PhD thesis (Mineralogy and Petrology of the New Idria the beginning of axial seafl oor spreading, and listric faulting along the District) is now considered a classic and involved a detailed fi eld axial trough was accompanied by the deposition of thick continental and laboratory study of the New Idria serpentinite massif and its clastics. The Tihama Asir complex has MORB affi nities similar to oce- tectonic inclusions. Bob’s thesis work showed that serpentinites are anic crust except that it was injected into old continental crust. More not igneous intrusives but instead are tectonically emplaced pieces work needs to be done to understand the transition from continental of ocean lithosphere. This work also laid the groundwork for his to oceanic crust. The problem that remains is what geochemical and/ seminal studies of ophiolites. or structural criteria can be used to distinguish between oceanic ridge and continental margin settings of ophiolites. Bob’s research interests include blueschist and eclogite metamor- phism of high-pressure/low-temperature terrains, the petrology of REFERENCES ophiolites, and the tectonics of accreted terrains. He has worked Coleman RG (1957) Mineralogy and Petrology of the New Idria District. in China, western North America, Oman, and Mongolia. He has Unpublished PhD thesis, Stanford University, Stanford, California, 166 pp also served as a consultant in environmental geology and has been Dilek Y, Furnes H (2014) Ophiolites and their origins. Elements 10: 93-100 involved in environmental studies on the asbestos Superfund site at New Idria, California. Bob was elected to the U.S. National Academy Goodenough KM, Thomas RJ, Styles MT, Schofi eld DI, MacLeod CJ (2014) Records of ocean growth and destruction in the Oman–UAE ophiolite. of Sciences in 1980. Elements 10: 109-114 Ishizuka O, Tani K, Reagan MK (2014) Izu-Bonin-Mariana forearc crust as a modern ophiolite analogue. Elements 10: 115-120 Pearce JA (2014) Immobile element fi ngerprinting of ophiolites. Elements 10: 101-108 Yang J-S, Robinson PT, Dilek Y (2014) Diamonds in ophiolites. Elements 10: 127-130

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