Paul C. Ragland

The Bataan orogene: Eastward subduction, tectonic rotations, and volcanism in the western Pacific (Philippines)

Abstract The Philippine mobile belt represents a crustal fragment, wedged between two subduction systems exhibiting opposite polarity. The eastern (Philippine—Quezon) system probably originated in the Eocene during northwest—southeast spreading of the west Philippine basin. Westward subduction is continued, probably as a result of northward motion of the Philippine basin crust. The western (Manila—Bataan) system originated in the Oligocene by spreading and formation of the South China Sea basin. Eastward subduction dominates the tectonics in the northern part of the archipelago and resulted in the formation of the Bataan orogene, a sequence of three parallel volcanic arcs emplaced in obducted oceanic crust. Geochemical and radiometric data indicate that the arcs migrated eastward with time (Miocene to Present) while changing composition from tholeiitic via calc-alkaline to shoshonitic. Centers of the latter two types are presently active. Depocenters behind the arcs also migrated eastward with time, suggesting correction of the isostatic disequilibrium caused by geanticlinal uplift of the orogene. Paleomagnetic evidence suggests that central Luzon is rotating counterclockwise probably due to differential spreading in the South China Sea basin. The west Philippine basin rotates clockwise. This results in significant “Einengung” in the southern part of the archipelago.

Juxtaposed Mesozoic diabase dike sets from the Carolinas: A preliminary assessment

Aeromagnetic anomaly maps of the North Carolina Piedmont provide confirming evidence of juxtaposed Mesozoic diabase dike sets. Dikes in North Carolina may be divided into four domains, on the basis of dike orientations. Most dikes are olivine normative, but there are apparent systematic differences in chemical compositions that are reflected in different orientations and ages. Two orientations dominate: northwest and north-south. The northwest-trending set dominates in the South Carolina Piedmont, and the north-south set extends through much of the central Virginia Piedmont and under the Coastal Plain in South Carolina. The north-south dikes are part of a set that converges near Charleston and Georgetown, South Carolina, and diverges northward into Virginia. Preliminary information suggests that the north-south dikes are younger than the more common northwest-trending dikes. Current models proposed for stress regimes associated with the breakup of Pangea can accommodate the northwest set, but not the north-south convergent set. This convergent set may be evidence for crustal deformation above a mantle plume centered near Charleston and Georgetown in Middle Jurassic time. It is possible that presumed mafic plutons under the Coastal Plain near Charleston, regional seismicity patterns, and orientations of the diabase dikes are all related to an intersection of zones of crustal weakness inherited from mid-Mesozoic time. The western margin of northwest-trending dikes may be related to tectonic features inherited from pre-Mesozoic time.

An example of trondhjemite genesis by means of alkali metasomatism: Rockford Granite, Alabama Appalachians

A model for trondhjemite genesis is proposed where granite is transformed to trondhjemite via infiltration by a Na-rich metamorphic fluid. The Rockford Granite of the Northern Alabama Piedmont serves as the case example for this process and is characterized as a synmetamorphic, peraluminous trondhjemite-granite suite. The major process operative in the conversion of granite to trondhjemite involves cation exchange of Na for K in the feldspar and mica phases through a volatile fluid medium. Whole-rockδ18O values for the trondhjemites are negatively correlated with atomic proportion K/Na ratio indicating a partial reequilibration of the altered granitoids with a Na- and18O-rich metamorphically derived fluid. Biotite decomposition to an Al-epidote-paragonitic muscovite-secondary quartz assemblage is also associated with the sodium metasomatism, as are apatite replacement by Al-epidote and secondary zircon crystallization. Rare albitization of primary magmatic plagioclase and discontinuous grossularite reaction rim growth on magmatic garnet are present in the trondhjemites indicating the mobility of Ca during alkali metasomatism. The replacement of magmatic phases by me tasomatic phases exemplifies the chemical changes produced during infiltration metasomatism where the trondhjemites are depleted in P2O5, Th, Rb, U, K2O, V, Sn, F, MgO, Pb, TiO2, FeO* and Li and enriched in CaO, Na2O, Zr and Sr relative to the granites. Other elements, such as Cr, MnO, Cu, Zn, Co, Ba, SiO2, Ni, Al2O3, are shown to be relatively immobile during the metasomatism. The infiltration metasomatism probably occurred during prograde regional metamorphism, when a discrete fluid phase was produced in the surrounding amphibolite-grade metasediments. Foliation planes in the granite apparently served as conduits for fluid flow with reaction-enhanced permeability accompanying the 8% molar volume reduction during Na-for-K exchange in the feldspars. A source for the Na and Sr in the metamorphic fluid may have been paragonitic muscovite in the metasedimentary country rocks. Rubidium and K were probably retained in metasedimentary biotite. The Silent Lake pluton in southeastern Ontario is a possible analogue to the alkali metasomatic processes affecting the Rockford Granite.

Temporal Chemical Variations Within Lowermost Jurassic Tholeiitic Magmas of the Central Atlantic Magmatic Province

The Central Atlantic Magmatic Province (CAMP), of greater extent than any other large igneous province (LIP) yet identified surrounds the Central Atlantic in eastern North America, northeastern South America, western Africa, and southwestern Europe. It covers over 7 x 10 9 km 2 and was active for no more than 4 Ma. Virtually all CAMP rocks are mafic tholeiites, and include both intrusives and extrusives. The most extensive intrusives, diabase (dolerite) dikes, occur in three main swarms on Pangaea: NW-, NE-, and NS trending. These mafic tholeiites can be classified based on their Ti contents into low-Ti (LTi), intermediate-Ti (ITi), and high-Ti (HTi). The NE swarm contains primarily the ITi magma type, whereas the NW swarm is heterogeneous and contains all three types. The N-S swarm contains highly evolved (high-Fe) quartz tholeiites in North America and ITi rocks in South America. These dike swarms can be correlated across the Atlantic basin on the basis of composition and attitude. The two principal magma types within the CAMP, LTi and ITi, were derived from mantle sources that were compositionally similar and contained both continental lithospheric and asthenospheric components. Compared with other large igneous provinces the CAMP basalts show depleted geochemical characteristics. Compositional differences between them are primarily due to differences in depth and degree of melting; LTi represents the deepest and greatest degrees of melting. The temporal progression of the chemical characteristics indicate deeper melting with time, which is consistent with a shallow (such as crustal thinning) and passive origin for the break-up of the Pangaean continent.

Recognition of contrasting magmatic processes using SB-systematics: An example from the western Central Luzon arc, the Philippines

Abstract The western Central Luzon volcanic system is associated with eastward subduction along the Manila Trench. Two arcs, the Bataan (BA) and the Mindoro (MA) arcs have resulted from this subduction and are separated by a NE-SW-oriented rift related zone of volcanism called the Macolod Corridor (MC). The BA consists of volcanism related to partial melting of the mantle wedge initiated by fluids from the subducted slab. The MA volcanics are also related to the subduction process but LILE and radiogenic Sr isotope data compared with the western part of the BA are significantly higher. The MC also has high LILE data compared with the western section of the BA but lower radiogenic Sr isotope values than the MA. Trace-element modelling has been used to explain SB-systematics (Sr/Ca vs. Ba/Ca) observed in geochemical data from volcanoes worldwide. These trends and their sense of curvature are most probably the result of variations in plagioclase and clinopyroxene crystal fractionation. SB-systematics have been examined from four volcanoes: Mt. Natib and Mt. Mariveles within the western section of the BA; Mt. Maquiling within the MC; and the Mt. San Cristobal-Banahaw Complex within the MA. The results suggest that crystal fractionation was a dominant process during the evolution of the magma chambers below the various volcanoes. In addition, initial melt compositions suggest that the source of the MA magmas has been enriched in a crustal component. The original magma composition(s) from the MC volcano, Mt. Maquiling, appears to have been generated by smaller degrees of partial melting compared with the BA volcanoes studied.

Trace elements in continental-margin magmatism: Part I. Trace elements in the Clarno Formation of central Oregon and the nature of the continental margin on which eruption occurred: Summary

The Clarno Formation is an Eocene to lower Oligocene volcanic and volcaniclastic formation in central Oregon. It was apparently formed in response to subduction of Pacific Ocean crust under the western margin of North America. Clarno flow rocks, which constitute at least 50% of the formation, are compositionally intermediate between flow rocks typical of continental margins and those typical of island arcs. The dominant rock type is andesite, in a range from rhyolite to basalt, and the SiO 2 frequently distribution is unimodal about a mean of 60%. The principal phenocryst in the basalts and andesites is clinopyroxene. Abundances of lithophile elements (K, Rb, and Ba) in the basalts are very similar to average abundances in calc-alkalic basalts of island arcs. Lithophile abundances in Clarno andesites, however, are generally greater than those of island-arc andesites and less than those of andesites from continental margins. The Clarno Formation may be inferred to have formed on a crust intermediate between oceanic and continental; a thickness of 20 to 30 km can be estimated from K 2 O contents and also from Rb-Sr relationships.

Use of principal components analysis in petrology: an example from the Martinsville igneous complex, Virginia, U.S.A.

SummaryThe Martinsville igneous complex is located in the Smith River allochthon, within the Piedmont of southwestern Virginia, U.S.A. This Ordovician complex consists of two main plutonic units: the mafic Rich Acres suite and the Leatherwood Granite. Four lithologic phases can be recognized in the Rich Acres and two are present in the Leatherwood. Major- and trace-element analyses from these six phases have been examined by principal components analyses (PCA); the first two principal components account for 86.9 percent of the total variance in the database, as opposed to about 35 percent for the first two original variables. Examination of variable loadings and sample scores for these two principal components has led to a number of observations about which original chemical variables best characterize the database. Mixing lines, control lines, and the “lever rule” can be used on bivariate PC plots as they can on bivariate plots of original chemical variables.Results of the PCA coupled with field and petrographic relationships allow for some hypotheses to be posed concerning petrogenetic relationships among the lithologic units. Among these hypotheses are 1) some type of mixing process occurred between the Leatherwood and Rich Acres; 2) the lithologic phases within the Rich Acres form one cogenetic suite, and 3) the Rich Acres and Leatherwood apparently are not comagmatic, in contrast to earlier suggestions. PCA can also be used to place constraints on different crystal-fractionation models. Results for PCA are compared with those for discriminant function analysis (DFA); PCA indicates a compositional continuum between most groups, whereas DFA shows large compositional gaps. The results for PCA seem to be closer to the true situation.ZusammenfassungDer magmatische Komplex von Martinsville liegt im Smith River Allochton innerhalb des Piedmont des südstlichen Virginia, USA. Der Komplex besteht aus zwei tektonischen Haupteinheiten: die matische Rich Acres Abfolge und der Leatherwood Granit. Radiometrische Daten für den letzteren zeigen Ordovicisches Alter. Auf der Basis von unterschiedlicher Mineralogie und Textur existieren in der Rich Acres Abfolge vier lithologische Phasen, und im Leatherwood Granit zwei.Die Spurenelemente dieser sechs Phasen wurden durch Hauptkomponenten Analyse (PCA) untersucht; die ersten beiden Hauptkomponenten machen 86,9% der totalen Varianz in der Datenbasis aus, im Gegensatz zu ungefähr 35% für die ersten zwei Originalvariablen. Untersuchung der variablen Gewichtungen und der Proben „Scores” für diese zwei Hauptkomponenten zeigen, welche ursprünglichen chemischen Variablen die Datenbasis am besten charakterisieren. Beispielsweise bestätigt PCA, daß MgO eine der wichtigen Variablen ist, die die Datenbasis charakterisieren, SiO2 jedoch nicht. Außerdem können Mischlinien, Kontrollinien und die Hebelregel auf Bivarianten von Hauptkomponenten benützt werden, ebenso wie auf bivarianten Plots die ursprüngliche chemische Variable darstellen:Hauptkomponentenanalyse, zusammen mit den Gelände- und petrographischen Beziehungen ermöglicht es, Hypothesen bezüglich der petrogenetischen Beziehungen zwischen den lithologischen Einheiten aufzustellen. Diese umfassen:1.einen Misch-Vorgang, sehr wahrscheinlich Magma-Mischung, zwischen dem Leatherwood und dem Rich Acres Komplex2.die vier lithologischen Phasen innerhalb des Rich Acres Komplex bilden eine cogenetische Suite und3.die Rich Acres und Leatherwood Komplexe sind offensichtlich nicht comagmatisch im Gegensatz zu früher vorgelegten Anregungen. Die PCA-Diagramme können auch dazu benützt werden, um verschiedene Kristall-Fraktionierungsmodelle einzuschränken. Die Ergebnisse von PCA werden mit denen der diskriminanten Funktionsanalyse (DFA) verglichen; PCA weist auf ein Kontinuum der Zusammen setzungen zwischen den meisten Gruppen hin, während DFA große Lücken im Spektrum der Zusammensetzungen erkennen läßt. Die Ergebnisse für PCA scheinen der tatsächlichen Situation näher zu kommen.

Geochemical evolution of the Precambrian Old Rag Granite, Virginia, U.S.A.: testing a UTh exploration model

Abstract The Old Rag Granite is one of several recognized units within the Virginia Blue Ridge Complex generally composed of Grenville-age gneisses and granitoid intrusive rocks. Chemical variation in the Old Rag Granite indicates simple orthomagmatic crystallization from the borders to the core of the pluton. Fractional crystallization of plagioclase, oxides, apatite and zircon accounts for much of the variation. Some trace elements, however, show anomalous behavior in the central part of the pluton. Y and Sr (and possibly Ba and Nb) are enriched whereas Rb and total REE are depleted in the central part of the pluton. Late crystallization of monazite could contribute to the observed REE patterns. It could not, however, account for Rb depletion and Sr enrichment, and no clear relationship between REE data and Th/U ratios are observed. The geochemical patterns indicate overall orthomagmatic crystallization overlain by a late-stage hydrothermal event. An exploration model for uraniferous granites, based upon the uranium deposits at Rossing and Bokan Mountain, included the Precambrian basement of central and northern Virginia in a list of proposed target areas. Geological, mineralogical and whole-rock chemical characteristics show a close association between the Old Rag Granite and the Rossing-type deposits. The uranium (mean 6.9 ppm, range 0.1–19.8 ppm) and thorium (mean 43 ppm, range 3.5–114 ppm) values are anomalous. Th/U ratios average 7.5 and are widely variable, indicating a decoupling of uranium from thorium. Neither U nor Th are lognormally distributed within the Old Rag Granite, requiring an explanation beyond simple removal of U by weathering. Rayleigh fractionation modeling shows that both U and Th, although probably distributed originally by closed-system fractionation, have been subsequently mobilized. Uranium can be lost in an oxidized form by secondary processes such as weathering or hydrothermal alteration. Thorium, however, will be affected only by hydrothermal processes. It is concluded that although primary fractionation was orthomagmatic, the Old Rag Granite was affected by late hydrothermal alteration whereby U was lost from the body and Th was redistributed. Three primary avenues for U migration are possible: (1) the rocks of the Saddleback Mountain Intrusive Suite which may have been contemporaneous with the Old Rag Granite; (2) Paleozoic fault zones that cut the Old Rag pluton; and (3) quartz veins, pegmatites, cupolas and roof pendants of the country-rock Nellysford gneiss that have been lost to erosion. The last option is preferred on geological and geochemical grounds. It is possible that the sedimentary uranium deposits of the adjacent Triassic Culpepper Basin were derived from eroded upper levels of the Old Rag Granite and like intrusives of the Virginia Blue Ridge Complex.

An example of trondhjemite petrogenesis: the Blakes Ferry pluton, Alabama, U.S.A.

Abstract The Blakes Ferry pluton is a calc-alkaline pluton that is exposed in southwestern Randolph County, Alabama, U.S.A. The petrogenesis of the Blakes Ferry pluton has been a controversy for almost 15 years. A petrogenetically coherent model is presented, however, as a result of new mineral analyses and the examination of previous analytical data. Crystal fractionation of plagioclase has been substantiated based on mass-balance calculations. Assimilation-crystal fractionation modelling and mixing calculations suggest that the biotite-muscovite-sphene-rich schlieren observed within the Blakes Ferry pluton are remnants of partially assimilated metasedimentary rock. Furthermore, the biotite chemistry of the Blakes Ferry pluton is quite similar to that of the biotites in the Wedowee schists which are the host rock to the Blakes Ferry intrusion. The above indicates the assimilation-mixing of the Wedowee metasediments occurred toward the end of plagioclase fractionation. The Blakes Ferry pluton has both l-and S-type characteristics interpreted as inherited from the source melting of an igneous rock with subsequent assimilation of the metasediments surrounding the pluton. The Elkahatchee Quartz Diorite, a large older tonalitic intrusive near the Blakes Ferry pluton, has been ruled out as a potential source rock contrary to previous interpretations. Crystal fractionation of a more mafic magma (e.g., gabbro) to generate the Blakes Ferry melt also does not seem plausible because the associated sequence of differentiated products are not observed in the region. Based on REE modelling and experimental work, the only plausible source is the partial melting of a MORB with a hornblende-quartz eclogite residual. The most likely source for the partial melting of a MORB is in a subduction related environment. Several researchers have proposed an arc setting for associated rocks within the same structural and metamorphic block that the Blakes Ferry poluton is situated. This suggests that the necessary setting for the partial melting of MORB was present in the region. The partial melting of approximately 28% MORB coupled with the generation of 72% hornblende-quartz eclogite residual in a subduction zone is envisioned. The density differences between the trondhjemitic melt and the surrounding mantle generated the ascent of the magma. Plagioclase fractionation via filter pressing probably occurred during ascent. The magma stalled in the Wedowee metasediments because the density contrasts no longer existed between melt and surrounding rocks. Assimilation and further fractional crystallization took place as the magma cooled to become the trondhjemitic schlieren-rich body observed in the field today.

Silica standardization: a discriminant technique applied to a volcanic arc system

Abstract A classic tholeiitic, calc-alkaline, and shoshonitic island-arc sequence has developed in western Luzon, the Philippines, as a result of subduction defined by an eastward-dipping Benioff zone. A simple technique of standardizing the data allows chemical concentrations to be recast to “andesite equivalent” compositions. Based on the excellent linear correlations that exist between SiO2 and other elements, individual analyses were standardized to 55% SiO2 by projecting parallel to least-squares regression lines. Raw and SiO2-standardized data were then treated by an univariate one-factor analysis of variance and a multivariate discriminant function analysis. Results of the analysis of variance indicate that at the 99% confidence level; 20 of the 27 chemical variates discriminate between at least two of the three geographically distinct magma series; SiO2-standardized data generally discriminate better than do raw data; and K2O, Ba, Rb, Sr, and Zr provide the best discrimination. Better classification of samples into the appropriate series is obtained from results of the discriminant function analysis using the SiO2-standardized data rather than results using the raw data. With this standardizing technique it was possible to detect offsets of SiO2-standardized K2O isopleths along the Manila Bay fracture zone that correspond to offsets in contours of depths for earthquake foci.