James E. Quick

Ductile deformation and the origin of layered gabbro in ophiolites

Layered gabbros in ophiolites have been commonly interpreted in terms of crystallization beneath oceanic spreading centers in magma chambers up to 30 km wide and 3–6 km deep. Although large, steady state magma chambers provide a possible explanation for the limited diversity of mid-ocean ridge basalts, their existence is not supported by geophysical observations. Sinton and Detrick (1992) recently reviewed seismic data from oceanic spreading centers and concluded that there is little evidence for steady state magma chambers beneath slow spreading ridges and that fast spreading ridges appear to be underlain by small (1–4 km wide) magma chambers perched on large (∼8 km wide) zones of low-velocity material, which are interpreted to consist of partially molten crystal cumulates. In this paper, we compare the geology of the gabbroic rocks of the Samail ophiolite, which is thought to have formed by fast spreading, with results of a finite element analysis of the Sinton-Detrick model for fast spreading centers. Our results demonstrate that flow, similar to that postulated by Sleep [1975], within a thick zone of cumulates during crustal extension could produce orientations of foliation and layering approximating those observed in the Samail gabbros. As a consequence of this deformation, (1) layers deposited on the floor of the magma chamber will be deformed and rotated into upwardly concave shapes that dip toward the spreading center, (2) the gross fabric defined by these orientations may greatly exceed the dimensions of the deforming regime, let alone the magma chamber; (3) feeder dikes that penetrate the cumulate pile near the spreading axis will be rotated into parallel with the layering, but those that invade farther from the ridge will be less deformed and retain crosscutting relationships; and (4) total deviatoric strain should increase down section toward the Moho so that primary magmatic structures and textures will be modified or obliterated in the lowermost gabbros. Upward migration of melts through the subsiding and deforming crystal mush may damp out cryptic varition in the gabbros and help maintain the magma chamber in a compositional steady state.

Emplacement dynamics of a large mafic intrusion in the lower crust, Ivrea‐Verbano Zone, northern Italy

The Ivrea-Verbano Zone of northern Italy provides an opportunity to study directly the effects of intrusion of large volumes of mantle-derived melts into the lower continental crust. Alpine uplift has exposed a complex of mafic to intermediate plutonic rocks, which had intruded the lower continental crust during the Permian. Field mapping reveals a gross arcuate structure within the complex that has survived tilting and uplift. In terms of the orientation during intrusion, banding and foliation are subhorizontal near the base of the complex and steepen gradually up section. Outcrop relations indicate that the cumulates were profoundly affected by extensional deformation while they still contained interstitial melts. The intensity of this hypersolidus deformation increases downward in the complex. The above characteristics are explained in terms of a model in which a huge volume of cumulates (8 km thick and >20 km long) crystallized from a small (≤1 km thick and ≤4 km wide), periodically refilled magma chamber which remained at a relatively fixed position through time. Large-scale ductile deformation of the complex and transport of cumulates downward and outward from the magma chamber occurred as a consequence of extensional tectonics. This process is capable of generating large sheets of gabbro in the lower crust. Gabbro complexes formed in this manner would be characterized by (1) widespread evidence of synmagmatic deformation dominated by stretching, (2) downward increasing intensity of deformation, (3) upward steepening layering and foliation that is concave toward the center of spreading, and (4) a scarcity of crosscutting intrusive relationships.

Chemical evolution of a large mafic intrusion in the lower crust, Ivrea-Verbano Zone, northern Italy

The Ivrea-Verbano and adjacent Strona-Ceneri zones have been described collectively as a section through the continental crust. While resident in the lower crust, amphibolite to granulite-facies paragneiss of the Ivrea-Verbano Zone was intruded by huge volumes of mafic to intermediate plutonic rocks grouped as the Mafic Complex. Growth of the Mafic Complex involved hypersolidus deformation in an extensional environment. Isotopic and trace element variations close to the axis of this structure indicate crystallization from mantle-derived melts that were extensively contaminated by crustal material. Previous investigations determined that the contaminant was fingerprinted by 87Sr/86Sr > 0.71, δ18O = 10–12.5‰, and a positive Eu anomaly. In the present study, the contaminant is also shown to have been enriched in Ba with respect to Rb and K. Charnockites associated with paragneiss septa in the lower part of the Mafic Complex have the appropriate chemistry to be samples of the contaminating material. These chemical features can be explained by melting of granulite-facies paragneiss, which had previously been depleted in K and Rb by an earlier melting event. The Ba enrichment in the core of the Mafic Complex can be modeled by a replenishment-tapping-fractional-crystallization (RTF) process operating within a small magma chamber is repeatedly replenished by mantle melts and contaminated by Ba-rich charnockite. Very high Ba/K in the lower part of the complex are tentatively attributed to chemical exchange between the cumulate framework and infiltrating anatectic melts from underlying paragneiss septa. In contrast to the Mafic Complex, the chemistry of coeval granites in the adjacent Strona-Ceneri zone reflect a component derived from crustal rocks that had not been significantly depleted by a previous melting event. Significantly, the incompatible trace element abundances in the Mafic Complex and Strona-Ceneri granites are similar to model compositions for the lower and upper crust, respectively.

Synmagmatic deformation in the underplated igneous complex of the Ivrea-Verbano zone

The Ivrea-Verbano zone, northern Italy, contains an igneous complex up to 10 km thick that is thought to have been intruded near the interface between the continental crust and mantle during the late Paleozoic. New data indicate that this complex is pervasively deformed and concentrically foliated. Widespread deformation under hypersolidus conditions is indicated by growth of undeformed poikilitic phases across the foliation and segregation of late-stage melts into high-temperature faults and pressure shadows of boudins. The presence of analogous features in ophiolitic gabbros suggests that emplacement of the Ivrea- Verbano zone plutonic rocks involved large-scale flow of crystal mush in a dynamic, and possibly extensional, tectonic environment.

Late Proterozoic transpression on the Nabitah fault system—implications for the assembly of the Arabian Shield

Abstract The longest proposed suture zone in Saudi Arabia, the Nabitah suture, can be traced as a string of ophiolite complexes for 1200 km along the north-south axis of the Arabian Shield. Results of a field study in the north-central shield between 23° and 26°N indicate that the Nabitah suture is indeed a major crustal discontinuity across which hundreds of kilometers of displacement may have occurred on north-south trending, subvertical faults of the Nabitah fault system. Although not a unique solution, many structures within and near these faults can be reconciled with transpression, i.e., convergent strike-slip, and syntectonic emplacement of calc-alkaline plutonic rocks. Transcurrent motion on the Nabitah fault system appears to have began prior to 710 Ma, was active circa 680 Ma, and terminated prior to significant left-lateral, strike slip on the Najd fault system, which began sometime after 650 Ma. Northwest-directed subduction in the eastern shield could have produced the observed association of calc-alkaline magmatism and left-lateral transpressive strike slip, and is consistent with interpretation of the Abt schist and sedimentary rocks of the Murdama group as relics of the associated accretionary wedge and fore-arc basin.

Development of a deep-crustal shear zone in response to syntectonic intrusion of mafic magma into the lower crust, Ivrea–Verbano zone, Italy

Abstract A 1 to 1.5 km-thick, high-temperature shear zone is localized in wall rocks subparallel to the eastern intrusive contact of the Permian Mafic Complex of the Ivrea–Verbano zone (IVZ), Italy. The shear zone is characterized by concentrated ductile deformation manifested by a penetrative foliation subparallel to the intrusive contact and a northeast-plunging sillimanite lineation. Evidence of noncoaxial strain and transposition is widespread in the shear zone including such features as rootless isoclinal folds, dismemberment of competent layers, and scattered kinematic indicators. The metasedimentary rocks in the shear zone are migmatitic, and the accumulation of leucosome is variable within the shear zone. Near the intrusive contact with the Mafic Complex leucosome forms ∼20 vol% of the wall rock, whereas leucosome concentrations may locally reach ∼60 vol% of the wall rock near the outer limits of the shear zone. This variation in vol% leucosome suggests melt/magma migration from the inferred site of anatexis along the intrusive contact to lower-strain regions within and near the margins of the shear zone. The leucosome accumulations chiefly occur as layer-parallel concentrations, but are also folded and boudined, and locally are associated with tension gashes and fracture arrays. Networks of granitic dikes and small plutons in the eastern IVZ suggest that some magmas migrated out of the high-temperature shear zone. Some magma apparently migrated laterally along the strike of the shear zone and concentrated in areas of lower strain where the intrusive contact takes a major westward bend. The high-temperature shear zone is interpreted as a “stretching fault” (or stretching shear zone) after Means [W.D. Means, Stretching faults, Geology 17 (1989) 893–896], whereupon the metasedimentary wall rocks and associated leucosome deformed synchronously with the multistage emplacement and deformation flow of the Mafic Complex. The recognition of a high-temperature shear zone associated with the emplacement of mafic igneous rocks into the deep crust is an example of the progressive stratification of the lower crust during magmatic under- or intraplating that has consequences for seismic imaging and its interpretation.

Pressure Gradient and Original Orientation of A Lower‐Crustal Intrusion in the Ivrea‐Verbano Zone, Northern Italy

The southern Ivrea‐Verbano Zone of the western Italian Alps contains a huge complex of mafic to intermediate plutonic rocks that intruded the lower continental crust during the Permian. Recent geologic mapping of the complex has characterized the effects of Alpine deformation and has demonstrated that the complex contains an arcuate internal structure. Building on this geologic foundation, we report a thermobarometric study of the plutonic and associated metamorphic rocks that was performed to better constrain the depth and orientation of the complex at the time of intrusion. The results demonstrate a continuous increase in equilibration pressure from 5 ± 1 kb along the eastern intrusive contact of the complex to 8 ± 1 kb near the western limit of the complex. After correcting for the effects of Alpine faulting, the observed pressure gradient ranges from 0.32 to 0.38 kb/km. Given the large uncertainties inherent in the geobarometric calculations relative to the narrow pressure range recorded in the complex, we conclude that the observed gradient is indistinguishable from a normal, vertical pressure gradient in the lower crust. It appears that, following intrusion and equilibration at a depth of 15 to 25 km, the complex was uplifted and rotated approximately 90°. The data also demonstrate that the internal arcuate structure formed in the complex before the observed pressure gradient was established. This result reinforces models for the growth of the complex by synmagmatic deformation and large‐scale necking during crustal extension and excludes the possibility that the arcuate structure was produced during the more recent Alpine uplift.

Density-controlled assimilation of underplated crust, Ivrea-Verbano zone, Italy

Abstract Structural, chemical and density data from the southern Ivrea-Verbano Zone indicate that the evolution of the lower crust may be strongly influenced by evolving density contrasts between mafic magmas and melting country rocks. The Ivrea-Verbano Zone contains a 10 km thick igneous mafic complex that intruded high-grade metamorphic rocks while they were resident in the lower crust. Heat released from the mafic intrusion induced partial melting in the country rocks on a regional scale. Slivers of crustal rocks (septa) are interlayered with igneous mafic/ultramafic rocks deep in the complex and show evidence of an advanced degree of partial melting. The chemical and isotopic composition of the Mafic Complex indicates significant contamination of mantle magma by a component delivered from a crustal source depleted in granophile elements, similar to the septa. In contrast, the present roof rocks cannot represent the source of the main contaminant because they are too rich in incompatible elements. The computed density of the mafic melt at the pressure conditions of the intrusion is intermediate between measured densities of the (lighter) roof rocks and the (heavier) septa. It appears that removal of buoyant anatectic melts increased the density of a restite layer on the roof. After the density exceeded that of the mafic melt, the restite layer was incorporated into the growing igneous body, creating a septum and providing the appropriate source for the contaminant of the Mafic Complex. In the Ivrea-Verbano Zone this process was episodically repeated during the evolution of the Mafic Complex. Worldwide, this process may profoundly influence the chemistry of continental basalts and may create a dense and layered lower crust.

Significance of Melt-Wall Rock Reaction: A Comparative Anatomy of Three Ophiolites

The ascent of basaltic melts through the upper mantle results in chemical disequilibrium between the melts and the pyroxene and plagioclase of the wall-rock peridotite. Phase and cryptic variations in ophiolitic peridotites demonstrate that the resulting reactions deplete the mantle in magmatophile components and enrich ascending melts in Ca, Na, Al, and incompatible trace elements while buffering their Mg/Mg + Fe ratios at primitive values (>0.6). A comparative anatomy of the Trinity, Oman, and Darb Zubaydah ophiolites illustrates both the significance of this process in shaping the composition of the shallow lithospheric mantle and how the integrated effects may reflect tectonic setting. A first-order correlation between crustal thickness and degree of mantle depletion exists, but multiple rifting events may remove part of the crustal record so that melt/rock ratios are difficult to quantify. Most impressive in ophiolitic peridotites is the abundance of melt crystallization and reaction products in zones of focused porous flow and(or) conduits indicating that melt segregation occurs at depths >20 km in extensional tectonic settings. Within these zones of melt transport, melt/rock reaction ratios will vary as the composition of the wall rocks evolves with melts becoming chemically insulated from the wall rocks when reaction zones of dunite develop. This may help explain why many MORBs retain trace-element signatures of a deep garnet-bearing source.

Hydrogen and Oxygen Isotope Constraints on Hydrothermal Alteration of the Trinity Peridotite, Klamath Mountains, California

The Trinity peridotite represents a rare opportunity to examine a relatively fertile plagioclase peridotite that was exhumed and later subjected to intrusive events in a seafloor environment, followed by its emplacement and incorporation into a continent. Over 250 stable isotopic determinations on whole rocks and minerals elucidate the hydrothermal evolution of the Trinity complex. All three serpentine polymorphs are present in the Trinity peridotite; these separate on the basis of their δD values: antigorite, -46 < δD < -82‰ and lizardite and chrysotile, -90 < δD < -106 and -110 < δD < -136‰, respectively. Antigorite coexists with chlorite, talc, and tremolite in contact aureole assemblages associated with Silurian/Devonian gabbroic plutons. Lizardite and chrysotile alteration carries a meteoric signature, which suggests association with post-emplacement serpentinization, or overprinting of earlier low-temperature seafloor serpentinization. Regionally, contours of δD values exhibit bulls-eye patterns associated with the gabbroic plutons, with δD maxima coinciding with the blackwall alteration at the margins on the plutons. In contrast to the hydrogen isotope behavior, oxygen isotope values of the three polymorphs are indistinguishable, spanning the range 5.3 < δ18O< 7.5, and suggesting low integrated fluid fluxes and strongly 18O-shifted fluids. Inferred primary δ18O values for peridotite, gabbro, and late Mesozoic granodiorite indicate a progressive 18O enrichment with time for the source regions of the rocks. These isotopic signatures are consistent with the geology, petrochemistry, and geochronology of the Trinity massif, which indicate the following history: (1) lithospheric emplacement and cooling of the peridotite in an oceanic environment ~472 Ma; (2) intrusion of gabbroic plutons into cold peridotite in an arc environment between 435 and 404 Ma; and finally (3) intrusion of felsic plutons between 171 and 127 Ma, long after the peridotite was incorporated into the continental crust.