Zambales Ophiolite Complex Research Articles

Geology of the Southern Zambales Ophiolite Complex, (Philippines): juxtaposed terranes of diverse origin

Abstract The Zambales Ophiolite Complex, which is made up of three massifs—Masinloc, Cabangan and San Antonio, was generated in a subduction-related marginal basin. Combined field geological and geochemical evidences show that the Cabangan and San Antonio massifs are genetically related to the Coto (transitional mid-ocean ridge-island arc) and Acoje (island arc) blocks of the Masinloc massif, respectively. A tectonic contact, the Subic Bay Fault Zone, is believed to separate the San Antonio and Cabangan massifs. The San Antonio massif is a displaced terrane rifted from the Acoje block and translated southward to its present position through the West Luzon Shear (?)/Subic Bay Fault Zone. The Zambales Ophiolite Complex could have not formed through the rifting or folding of the arc-related Acoje block-San Antonio massif to generate the transitional mid-ocean ridge-island arc like Coto block-Cabangan massif!

Varying mantle sources of supra-subduction zone ophiolites: REE evidence from the Zambales Ophiolite Complex, Luzon, Philippines

The Zambales Ophiolite Complex, a supra-subduction zone ophiolite, is characterized by three volcanic-hypabyssal rock units: the Coto Block volcanic-hypabyssal rocks, the Coto dikes intruded into the Coto residual peridotites and the Acoje Block volcanic-hypabyssal rocks. The first two groups exhibit transitional mid-ocean ridge-island arc characteristics while the latter reveals island arc affinity. Furthermore, these three volcanic rock suites are characterized by differing bulk REE, major-element, trace-element and mineral chemistries. The Coto Block volcanic-hypabyssal rocks [ (Ce Yb) n 0.4–1.0; TiO 2 0.50–1.50 wt.%; Zr 31–76 ppm; Y 13–31 ppm] have higher REE, Ti, Zr and Y than the Coto dikes [ (Ce Yb) n 0.3–0.8; TiO 2 0.52–0.94 wt.%; Zr 15–55 ppm; Y 10–27 ppm] and the Acoje Block volcanic-hypabyssal rocks [ (Ce Yb) n 0.2–0.3; TiO 2 0.26–0.86 wt.%; Zr 11–45 ppm; Y 10–23 ppm] arguing for an increasing degree of melting of the sources from the former to the latter. These data suggest the involvement of several mantle sources which have undergone different degrees of partial melting and LREE addition as evidenced by the presence of LREE-enriched basalts [ (Ce Yb) n 3–5] among the Coto Block volcanic-hypabyssal rocks. These results further illustrate the complexities involved in the generation and evolution of supra-subduction zone ophiolites.

Supra-subduction zone ophiolites as favorable hosts for chromitite, platinum and massive sulfide deposits

Supra-subduction zone ophiolites, as exemplified by the Zambales Ophiolite Complex, host extensive chromitite, volcanic-hosted massive sulfide deposits and, to a lesser degree, platinum-group minerals. In contrast, mid-ocean ridge basalt ophiolites are almost barren of economic mineral deposits. The high degree of partial melting, high water pressure, changes in oxygen fugacity, temperature, pressure and magma mixing could explain the chromitite deposits in zupra-subduction zone ophiolites. Platinum-group minerals are modeled to be derived from multi-stage melting events that also characterize supra-subduction zone ophiolites. The presence of capping rocks and the tectonics of emplacement are believed to be critical in the preservation of volcanic-hosted massive sulfides in ophiolites. Marginal basins (= SSZ ophiolites) are more easily emplaced than the large, open sea oceanic basin ophiolites which are usually subducted. Geochemical and tectonic controls point to supra-subduction zone ophiolites as more promising exploration targets.

A Cretaceous to Paleocene‐Eocene South China sea basin origin for the Zambales Ophiolite Complex, Luzon, Philippines?

Abstract The Eocene Zambales Ophiolite Complex that exhibits transitional mid‐ocean ridge basalt‐island arc tholeiite (MORB‐IAT) characteristics was formed in a subduction‐related marginal basin. The different surrounding marginal basins of the Philippines, namely, the South China Sea, Sulu Sea Basin, Celebes Basin and the West Philippine Basin have all been modeled to be of probable provenance of this ophiolite complex. Certain information (e.g. age, rock geochemistry, paleomagnetic rotations) and limitations, nevertheless, are inconsistent with the ophiolite complex being generated in these regions. Recent geophysical evidence suggests that the southwest sub‐basin of the South China Sea Basin is probably Cretaceous to Paleocene‐Eocene in age. This makes it possible to speculate that the Zambales Ophiolite Complex could have come from this sub‐basin. The present day rifting of the southern Izu‐Mariana arc can be taken as a modern day analog of this type of ophiolite generation.

Platinum-group element abundance and distribution in chromite deposits of the Acoje Block, Zambales Ophiolite Complex, Philippines

Platinum-group elements (PGE) occur in ore-grade concentration in some of the chromite deposits related to the ultramafic section of the Acoje Block of the Zambales Ophiolite Complex. The deposits are of three types: Type 1 - associated with cumulate peridotites at the base of the crust; Type 2 - in dunite pods from the top 1 km of mantle harzburgite; and Type 3 - like Type 2, but in deeper levels of the harzburgite. Most of the deposites have chromite compositions that are high in Cr with Cr/(Cr + Al) (expressed as chromium index, Cr#) > 0.6; high-Al (Cr# < 0.6) chromite deposits are relatively rare, occurring as interspersed orebodies in regions of Types 2 and 3. Sulfide-bearing Type 1 high-Cr deposits are PGE-enriched; the high-Al deposits have lowest Ir and Pd. Among the PGEs, Ir ranges from <20 to 550 ppb, Rh from <100 to 1100 ppb, Rh from 0.7 to 760 ppb, Pt from <1.0 to 5960 ppb and Pd from <0.5 to 8350 ppb. Chondrite-normalized (CN) PGE patterns are diverse, and include those with both negative slope (Ir > Pd, thought to be characteristic of PGE-barren deposits) and positive slope (Ir < Pd, characteristic of PGE-rich deposits). Iridium, Ru and Os commonly occur as micron-size laurite (sulfide) inclusions in unfractured chromite. Laurite and native Os are also found as inclusions in interstitial sulfides. Platinum and Pd occur as alloy inclusions (and possibly as solid solution) in interstitial Ni-Cu sulfides and as tellurobismuthides in serpentine and altered sulfides. Variability of PGE distribution may be explained by alteration, crystal fractionation or partial melting processes. Alteration and metamorphism were ruled out, because PGE contents do not correlate with degree of serpentinization or the abundance and type (hydroxyl versus non-hydroxyl) of silicate inclusions in chromite. Preliminary Os isotopic data do not support crustal contamination as a source of the PGEs in the Acoje deposits. The anomalous PGE concentrations in Type 1 high-Cr chromite deposits are attributed to two stages of enrichment: an early enrichment of their mantle source from previous melting events and a later stage of sulfide segregation accompanying chromite crystallization. High-Al chromite deposits which crystallized from basalts derived from relatively low degrees of melting owe their low PGE content to partitioning of PGEs in sulfides and alloys that remain in the mantle. High-Cr deposits crystallized from melts that were previously enriched with PGEs during early melting events of their mantle source; Pt and Pd ore concentrations (ppm levels) are attained by segregation of magmatic sulfides. The Acoje deposits indicate that ophiolites are a potential economic source of the PGEs.

Zambales ophiolite, Philippines I. Geology and petrology of the critical zone of the Acoje massif

The Acoje massif constitutes the northernmost massif of the Zambales Ophiolite Complex, an assemblage of relatively intact fragments of a Mesozoic oceanic lithosphere. The critical zone of the massif consists of interlayered dunite, harzburgite, lherzolite, wehrlite, pyroxenite and feldspathic units which grade upsection into gabrronorite and gabbro cumulates. This sequence occurs between highly deformed residual harzburgite and a 4 to 5.5 k thick cumulate and isotropic gabbro sequence. The critical zone can be subdivided into a 540 m thick ultramafic zone and a 390 m thick mafic (feldspathic) zone. The boundary between these zones is marked by the first appearance of cumulus plagioclase. The paragenetic sequence that best describes the cumulus mineralogy is chromite, chromite + olivine, chromite + olivine + orthopyroxene, chromite + olivine + clinopyroxene (± orthopyroxene), olivine + clinopyroxene + plagioclase (± orthopyroxene, ± amphibole). The lithologie and chemical (including ƒO 2) variation across the critical zone reflects multiple magma influx, assimilation of tectonized harzburgite, magma mixing, fractional crystallization, and intercumulus melt/crystal interactions in a sub-oceanic magma chamber. Therefore, this critical zone represents the basal cumulate sequence of the Acoje massif, and not the uppermost portion of the residual mantle. There is a general trend of decreasing of deformation from the bottom to the top of the critical zone. This is attributed to the rheological inhomogeneity of the partially crystallized basal cumulate sequence during deformation. Geothermometric estimates based on the Mg-Fe 2+ exchange between olivine and spinel indicate equilibration to subsolidus temperatures.

High chromium and high aluminum chromite deposits in the Zambales ophiolite complex, Luzon, Philippines: Origin and tectonic significance

High chromium and high aluminum chromite deposits in the Zambales ophiolite complex, Luzon, Philippines: Origin and tectonic significance