Metamorphic Wall Rocks Research Articles

Rare Earth Elements Geochemistry of Laowan Gold Deposit in Henan Province: Trace to Source of Ore-Forming Materials

The compositions of REE in quartz and pyrite from the main stage of the Laowan gold deposit in Henan Province and that in quartz from Laowan granite were determined by inductively coupled plasma-mass-spectrometry (ICP-MS) to trace the source of ore-forming materials. Meanwhile, the REE compositions of the deposit ore, granite and metamorphic wall rock were also considered for comparative studies in detail. The range of ∑REE of quartz and pyrite from the deposit ores is 4.18 × 10 −6 ∼ 30.91 × 10 −6, the average of ∑REE is 13.39 × 10 −6, and the average of ∑REE of quartz in the Laowan granite is 6.68 × 10 −6. There is no distinct difference of REE parameters between the deposit ore quartz and granite quartz. The quartz in gold deposit has the same REE particular parameters as quartzes from Laowan granite, such as δ Eu, δ Ce, (La/Yb) N and (La/Sm) N , partition degree of LREE to HREE, especially, the chondrite-normalized REE patterns, but no similarity to those from metamorphic wall rock, which shows that ore-forming hydrothermal fluid is mainly the fluid coming from the Laowan granite magma, rather than metamorphic fluid. Meanwhile, comparison studies on REE features between minerals from the deposit ores and related geological bodies in the deposit show that REE characteristics of minerals can serve as an indicator of ore-forming fluid properties and sources, while the REE characteristics of the bulk samples (such as deposit ores, granites and wall rocks) can not trace the source of the ore-forming materials exactly.

Thermal Relaxation of a Magmatic Arc Complex, Mount Barcroft Area, Eastern California: 40Ar/39Ar Geochronologic Study of Hornblende and Biotite from Igneous and Metamorphic Rocks

The central and southern White Mountains constitute a well-exposed section of the middle to late Mesozoic convergent margin of western North America. New 40Ar/39Ar analyses were obtained for 8 Ca-amphibole and 13 biotite specimens from igneous and metamorphic wall rocks (8 rock samples from the Barcroft pluton, a single McAfee Creek-type aplite, 7 mafic dikes/metadikes, and two Andrews Mountain quartzites). Most of the spectra are very complicated. Except for one relatively fresh and four apparently fresh diabase dikes, all 9 granitoids, the 2 metadiabasic dikes, and both contact metamorphic quartzites partially lost (or gained) argon during heating and/or deuteric-hydrothermal alteration over ~100 m.y. of igneous arc evolution. New granitoid ages are similar to—or younger than—published U/Pb ages for the Middle Jurassic Barcroft and the Late Cretaceous McAfee Creek plutons, and are similar to published K-Ar and 40Ar/39Ar ages of the Cottonwood/Beer Creek and other plutons in the central and southern White Mountains. Contact metamorphic ages of wall rocks approximate nearby granitoid emplacement ages. Mafic dikes invaded the area episodically; most yield disturbed 40Ar/39Ar spectra exhibiting a range of cooling ages. Some, metamorphosed by the Barcroft pluton, must be older than 165 Ma; others give apparent ages of 144 (Independence dike swarm), 115-118, and 98 Ma. Clearly, multiple stages of mafic dike injection accompanied mid- and late-Mesozoic accretion of this sector of the North American continental margin.

Ductile–brittle transition along the Alaşehir detachment fault and its structural relationship with the Simav detachment fault, Menderes massif, western Turkey

Western Turkey has experienced extensional deformation since the Tertiary. An early phase of extensional deformation resulted in the formation of detachment faults in the northern part of the Menderes massif, the Alaşehir and Simav detachment faults. The footwalls of these two detachment faults are mid-crustal level rocks. The Alaşehir detachment fault developed in metamorphic rocks and in a syn-extensional intrusion, the Salihli granodiorite. The Salihli granodiorite shows a gradual change from undeformed isotropic granodiorite to protomylonite, mylonite and ultramylonite towards its upper structural levels. The deformed granodiorite, in turn, grades into a cataclastic zone whose uppermost part is the Alaşehir detachment surface. This gradual upward change from the undeformed granodiorite to a brittlely deformed detachment surface suggests that Tertiary extension resulted in a ductile deformation at depth and the ductilely deformed granitoids were brought to shallower depths where they were brittlely deformed. The metamorphic wall rocks of the intrusion are also mylonitized structurally upward. A similar transition has also been found in the footwall of the Simav detachment fault. The upper part of which is also a cataclastic zone leading to the Simav detachment fault separating the low-grade metamorphic rocks and/or non-metamorphic rocks in its hanging wall from mainly high-grade metamorphic rocks and syn-tectonic granitoids in its footwall. During ductile extension, greenschist facies-grade mylonitic deformation developed in the metamorphic rocks and the granitoids. Shear sense indicators along the two shear zones show top to N-NE sense shear, consistent with the regional Cenozoic extension direction in western Turkey. Available radiometric age data from the granitoids in the footwall of the two detachment faults suggest that the Cenozoic extension in western Turkey was initiated in Oligocene to Early Miocene and the Simav detachment fault is older than the Alaşehir detachment fault. Extensional features of these detachment faults suggest that the Simav detachment fault characterise the earlier stage of the Tertiary extensional tectonics of the Menderes massif, and later stage of this deformation resulted in the development of the Alaşehir detachment fault.

Geological features and origin of gold deposits occurring in the Baotou–Bayan Obo district, south-central Inner Mongolia, People's Republic of China

Located at western portion of northern margin of North China craton, the Baotou–Bayan Obo district is one of the most important Fe–REE–Nb and Au metallogenic provinces in China. Presently, about 52 gold deposits and prospects have been discovered, explored and mined, among which Shibaqinhao, Laoyanghao, Houshihua, Saiyinwusu, Wulashan and Donghuofang are the most important ones. All these gold occurrences can be subdivided into three groups (or types) according to its host rocks: (1) hosted by Archean high-grade metamorphic rocks; (2) hosted by Proterozoic sedimentary rocks; (3) hosted by or related to Hercynian alkaline intrusive rocks. The first group contains the Shibaqinhao, Laoyanghao and Houshihua gold deposits. Gold mineralization at these three deposits occurs within Archean amphibolite, gneiss and granulite as gold-bearing quartz veins and veinlet groups containing native gold, electrum, pyrite and chalcopyrite. The Saiyinwusu deposit belongs to the second group, and occurs within Proterozoic sandstone, quartzite and carbonaceous slate as quartz veins and replacement bodies along the fracture zones. Pyrite, marcasite, arsenopyrite, native gold and electrum are identified. The third group includes the Wulashan, Donghuofang and Luchang deposits. Gold mineralization at these three deposits occurs predominantly within the Hercynian alkaline syenite or melagabbro stocks and dyke swarms or along their contacts with Archean metamorphic wall rocks as K-feldspar–quartz veins, dissemination and veinlets. Pyrite, galena, chalcopyrite, native gold and calaverite are major metallic minerals. δ 34S value of sulfides (pyrite, galena and pyrrhotite) separates from groups 1 and 2 varies from −4.01‰ to −0.10‰ and −3.01‰ to 2.32‰, respectively. δ 34S values of Archean and Proterozoic metamorphic wall rocks for groups 1 and 2 deposits range from −20.2‰ to −17.0‰ and −15.8‰ to −16.2‰, respectively. The values are much lower than their hosted gold deposits. All these pyrite separates from Hercynian alkaline intrusions associated with the gold deposits show positive δ 34S values of 1.3‰ to 4.8‰, which is higher than those Precambrian metamorphic wall rocks and their hosted gold deposits. δ 34S values of the sulfides (pyrite and galena) from the Donghuofang and Wulashan deposits (group 3) increase systematically from veins (−14.8‰ to −2.4‰) to the Hercynian alkaline igneous wall rocks (2.8‰ to 4.8 ‰). All of these deposits in groups 1, 2 and 3 show relatively radiogenic lead isotopic compositions compared to mantle or lower crust curves. Most lead isotope data of sulfides from the gold ores plot between the Hercynian alkaline intrusions and Precambrian metamorphic wall rocks. Data are interpreted as indicative of a mixing of lead from mantle-derived alkaline magma with lead from Precambrian metamorphic wall rocks. Isotopic age data, geological and geochemical evidence suggest that the ore fluids for the groups 1 and 2 deposits were generated during the emplacement of the Hercynian alkaline syenite and mafic intrusions. The Hercynian alkaline magma may provide heat, volatiles and metals for these groups 1 and 2 deposits. Evolved metamorphic fluids produced by the devolatilization, which circulated the wall rocks, were also progressively involved in the alkaline magmatic hydrothermal system, and may have dominate the ore fluids during late stage of ore-forming processes. Most of these gold deposits hosted by Archean high-grade metamorphic rocks occur at or near the intersections of the NE- and E–W-trending fracture systems. The ore fluid of the group 3 deposits may have resulted from the mixing of Hercynian alkaline magmatic fluids and evolved meteoric waters. The deposits are believed to be products of Hercynian alkaline igneous processes along deep-seated fault zones within Archean terrain.

Mathematical analysis of rare earth element patterns of fluorites from the Ehrenfriedersdorf tin deposit, Germany: evidence for a hydrothermal mixing process of lanthanides from two different sources

Systematic changes in REE patterns of fluorites with various colourations from the Ehrenfriedersdorf tin deposit, Germany, can be related to the evolution of the mineral forming hydrothermal fluid within the endo- and exocontact of a related Li-F granite. One possible geochemical model that may explain the fluorite REE patterns assumes that lanthanides derived from two distinct element sources. It is suggested that a mantle- or lower crust-derived fluid (LREEs n < HREEs n ) participated as one source of elements in the formation of the Li-F granitic magmas and in ore deposition. The metamorphic wall rocks (LREEs n > HREEs n ) were involved as a second source of elements due to assimilation and fluid-rock interaction (including admixing of crustal fluids). Minerals that crystallised in the relatively cool and/or oxidising environment of the exocontact incorporated the REEs from the ore-forming fluid and therefore exhibit REE patterns that result from simple mixing of the lanthanide distributions of the two element sources. Mineral precipitation in the exocontact vein system led to a successive decrease of the total lanthanide concentration of the migrating fluid. The fluorite REE patterns further suggest that the mineral-forming fluid in the exocontact showed a relative enrichment of Eu that can be attributed to fluid evolution within the endocontact. The large ionic radius of Eu2+ hampered the incorporation of Eu into minerals crystallising from the fluid-magma system in this high temperature and/or low oxygen fugacity environment. This process lead to an enrichment of Eu in the fluid that subsequently entered the exocontact and resulted in a corresponding Eu depletion of the altered granite. The lanthanide distributions of three differently coloured fluorite samples from a single zoned fluorite aggregate out of a cassiterite-bearing quartz vein of the exocontact are suitable for mathematical modelling because of their sample location in a sequence of homogeneous wall rocks and their well-constrained relative age relationships. Mathematical modelling has been carried out to test whether the geochemical mixing model can indeed explain the entire REE patterns of the three samples. In accordance with the proposed geochemical model, it is assumed that two endmembers with distinct REE patterns were mixed and that a process such as mineral precipitation reduced the total REE content of the mixture. The incorporation of REEs from the fluid into fluorite is described by a partition coefficient. Model values that are in closest possible agreement with the measured fluorite lanthanide concentrations were derived from the mathematical model by minimising the mean squared deviation between the two data sets. It is shown that the global minimum was found by application of the Gauss-Newton method and a specially designed evolutionary strategy. Ce and Eu were not considered in a first analysis because positive anomalies in the fluorite REE patterns indicate anomalous geochemical behaviour of these elements during the ore forming processes. The differences between the model values and the measured lanthanide concentrations were found to be smaller than the analytical errors of the REE measurements. In a second analysis the Ce and Eu concentrations of the three fluorites were taken into account and deviations between the model and measured values for these two elements were observed that could not be explained by analytical errors only. These findings suggest that the geochemical mixing model is a likely explanation for the measured fluorite REE patterns, while the observed Ce and Eu contents do not simply result from mixing processes.

Translation and the resolution of the pluton space problem

The Late Cretaceous Mono Creek granite has a pronounced NW–SE elongate shape, 60 km long by 10 km wide, characteristic of plutons from the eastern Sierra Nevada batholith. An 8 km-wide bulge exists on the NE side of this pluton, which exhibits evidence of forceful emplacement (or in-situ ballooning), such as deflection of metamorphic wallrock and igneous foliation, and the orientation of fracture patterns. Three-dimensional strain analysis indicates that wallrock strains do not provide enough volume to accommodate the emplacement of the bulge, a recurring problem in studies of plutonic terranes. We suggest that emplacement of the Mono Creek bulge was accommodated by all components of the three-dimensional displacement field—including translation, rotation, and pure strain (shape change)—of the surrounding units. Classical strain analysis only addresses the rotation and pure strain components, and is incapable of quantifying the translation component. However, our analysis suggests that translation plays the dominant role in the emplacement process. A shell model of translation of the surrounding igneous and metamorphic units is proposed for the Mono Creek bulge, which suggests that the translation component decreases dramatically away from the intrusion, consistent with the observed geology and finite strain analysis. We propose that translation is the solution to the recurring pluton `space' problem, either through tectonically controlled (passive) or magmatically controlled (active) movement of the wallrocks. Translation is generally the neglected component of the displacement field, but it may often be evaluated through judicious use of finite strain analysis and tectonic reconstruction.

One-dimensional reactive transport models of alteration in the Tongchang porphyry copper deposit, Dexing District, Jiangxi Province, China

In the Tongchang porphyry copper deposit, the main hydrothermal alteration is most intense in and near the zone and gradually attenuates inward and outward, showing a symmetrical zoning with respect to the zone.Detailed field investigations of wall-rock alteration and ore-controlling fractures, combined with laboratory studies of lithology, mineral composition, and fluid inclusions, have led to an assessment of initial and boundary conditions and physical parameters such as species concentration, mineral volume fraction, temperature, porosity, reactive surface area, and flow velocity required for dynamic modeling of this system.We have developed a new program to model the coupled process of fluid flow involving advection and diffusion and chemical reactions incorporating mineral dissolution-precipitation, and have numerically simulated the spatial and temporal evolution of the systems for a duration of 100,000 yr. In the simulations, transient temperature fields are calculated for the main temperature range of ore formation from 200 degrees to 350 degrees C.Simulated mineral assemblages are generally in agreement with those observed in the real system. The important conclusions drawn from the combined researches of geology and geochemistry as well as dynamics of coupled fluid transport and reactions are as follows.Fluid flow and reactions were mainly controlled by the fractures striking around north-south and dipping to the west. In the exocontact zone, schistosity cleavages were also important fluid pathways in addition to the fractures. The formation of closely spaced fractures in the zone and especially in the exocontact zone by tectonic activity after potassium metasomatism and the reactive flow from the zone inward and outward were the principal reasons for the occurrence of the contact zone-symmetrical alteration zone.In the porphyry body, both the dissolution and precipitation of minerals were important; the rate of dissolution or precipitation of minerals gradually decreased in time. The nearer it was to the zone, spatially, the earlier the dissolution or precipitation of minerals was finished. In the exocontact metamorphic wall rock, the main processes were the transport and precipitation of SiO (sub 2 (sub (aq)) ) ; the rate of silification gradually decreased in time and space.In the alteration of porphyry body, Na (super +) was evidently removed from the alteration zone and Ca (super 2+) removed from the intense alteration zone to weak alteration zone, and the fluid was weakly acidic (pH [asymp] 4). In the metamorphic wall rock, SiO (sub 2 (sub (aq)) ) was mainly brought in from fluid. The evolution of SiO (sub 2 (sub (aq)) ) , Cl (super -) , Ca (super 2+) , and CO 3 (super 2-) was intimately correlated with alteration-mineralization intensity.

The Rb-Sr age and geological implication of alkaline syenitic complex from Liangtun-Kuangdonggou at Gaixian, eastern Liaoning Province

Liangtun-Kuangdonggou complex, 30 km southeast of Gaixian Town, Liaoning Province, is composed of aegirine-augite hornblende syenite, aegirine-augite syenite, hornblende syenite, pyroxene syenite and syeno-aegirinolite. Rb-Sr isotopic data to the complex are reported and used for discussion of chronology and geological implication. They show that the complex occurred in early Proterozoic and is a tectonic sign of extension setting. Moreover, the date indicates that the Liaohe Group, which is metamorphic wall rock of the complex, formed over 1 900 Ma ago.

The Liquiñe Ofqui fault zone: a long-lived intra-arc fault system in southern Chile

Plate reconstructions for the Cenozoic document relatively steady right-oblique subduction of the Farallon (Nazca) plate beneath the Chilean continental margin. The kinematic signature recorded along the intra-arc Liquiñe-Ofqui fault zone (LOFZ), a major feature of the southern Andes, may help to understand the way in which the Nazca-South America slip vector has been partitioned into strain and displacement along and across the continental margin. The LOFZ consists of two NNE-trending right-stepping straight lineaments, a strike-slip duplex at the right step, and curved features which splay off the straight lineaments toward the northwest. The LOFZ runs mostly through heterogeneously deformed Cenozoic plutonic rock of the North Patagonian Batholith and patchy metamorphic wall rock. Early Cenozoic volcano-sedimentary rocks and dyke swarms, found in close spatial association with the strike-slip duplex, are believed to have developed in strike-slip-related basins. Quaternary volcanoes are aligned parallel to the LOFZ. Both ductile and brittle kinematic indicators within centimeter-to-meter-wide high-strain zones, document late Cenozoic dextral shear deformation. Contrasting left-lateral deformation recorded on older and wider mylonitic zones, suggests that the LOFZ may be a long-lived shear zone that accommodated continental-scale deformation arising from the Farallon (Nazca)-South America plate convergence. A block rotation pattern, as indicated by paleomagnetic data, is consistent with the geometry and Cenozoic kinematics of the LOFZ.

ANATOMY OF A SEMIARID COASTAL SYSTEM: THE UPPER CARNIAN OF LOMBARDY (ITALY)

The mixed terrigenous-carbonate-evaporitic S.Giovanni Bianco Formation and dolomitic Campolungo Tongue (upper part of the Breno Formation), generally 2OO to 3OO m thick, are assigned to the Late Carnian. They respectively overlie lagoonal limestones (Gorno Fm.) and peritidal carbonates (Annunciata Member of the Breno Fm.), and underlie intraformational breccias and recrystallized limestones (Castro Fm.). Recognition of an unconformity, ascribed to a relative fall of sea-level (sequence boundary), allowed us to subdivide the Upper Carnian succession into two parts. ln the lower part (SGBL), six lithosomes were recognized. Red to green alluvial clastics in the south-east and south-west pass northward to mixed terrigenous-carbonate coastal sediments and finally to dolostones deposited in carbonate tidal flats. In the proximal sections of the Brescia Prealps, renewed north-westward progradation of alluvial redbeds with intercalated calclithite conglomerates points to a stage of teconic uplift. A distinct increase in quartz, representing a regional petrographic marker followed all across Lombardy, indicates deepening of erosion into the metamorphic wallrocks of the volcanic belt. A major hiatus at the top of the SGB1 is best documented in the northern Presolana area by a silcrete crust directly overlying the Julian Annunciata Member of the Breno Formation. In the Brembana Valley area, the discontinuity occurs within a greenish siliciclastic coastal plain succession, and may be traced at the top of a marker interval of interbedded reddish siltstones and sandstones. The upper part (SGB2) consists of four lithosomes. Greenish sandstones and siltstones, accumulating in coastal plains in the south-west, passed northward to mudrocks and dolostones. In the southernmost Camonica Valley area, mudrocks are locally interbedded with calcarenites containing bored or pedogenized lithoclasts ripped from the underlying sequence and varied bioclasts, restifiying to relatively open shallow-marine conditions during transgression. Next, thick gypsum accumulated in coasral salinas barred by locally oolitic platform carbonates to the north. Rare sandstone lenses occurring in the Brembana Valley at the top of the unit conrain exclusive rhyolitic detritus, indicating either a terminal phase of explosive volcanism or erosion of older felsic volcanic products.

Isotopic systematics of Pb/U (zircon) and 40Ar/39Ar (biotite-hornblende) from rocks of the central Foothills terrane, Sierra Nevada, California

Pb/U zircon and ^(40)Ar/^(39)Ar biotite-horn-blende studies in conjunction with detailed structural and textural studies clarify the age and structural relations of a unique cluster of plutons in the central Sierra Nevada western metamorphic belt. The plutons range from gabbro to tonalite/trondhjemite and include the Guadalupe Igneous Complex. Magmatic emplacement ages for the plutons are for the most part between ∼150 and ∼145 m.y., as recorded in both the zircon and hornblende isotopic systems. Additional ages of 138 and 123 m.y. were determined on small plutons having key structural settings. In general, the Pb/U and ^(40)Ar/^(39)Ar isotopic systems are well behaved. Second-order complications are locally present in the Pb/U systems, owing to the entrainment of older zircon from wall rocks, and in ^(40)Ar/^(39)Ar systems, owing to minor compositional differences in mica and amphibole phases. The age data and structural relations of the plutons in conjunction with ^(40)Ar/^(39)Ar metamorphic wall-rock biotite and hornblende age data constrain regional ductile deformation in the study area to have occurred between ∼150 and ∼125 Ma. Such deformation was accompanied by upper greenschist- to lower amphibolite-facies metamorphism and is at least in part closely related to pluton emplacement. Such deformation and metamorphism has in the past been attributed to the Late Jurassic Nevadan orogeny. This study shows clearly, however, that ductile deformation and metamorphism in the study area are post-Nevadan if modern concepts of the Nevadan orogeny are strictly adopted.

Geology, geochemistry, and tectonic setting of the Ben Ghnema batholith, Tibesti massif, southern Libya

The Precambrian Tibesti massif of southern Libya and northern Chad is separated from other massifs to the east and west by relatively deep basins of Paleozoic and younger sedimentary rocks. It is also bordered on the north by a northward-thickening wedge of Phanerozoic sedimentary rocks and connects southward, under shallow sedimentary cover, with the older Precambrian terranes of central Africa. The northwestern part of the Tibesti massif contains the Ben Ghnema batholith, a complex assemblage of plutons intruded along a north-south axis between metamorphic wall rocks. To the west, the wall rocks are felsic, platformal, and metasedimentary; to the east is a more intensely folded, mafic basinal sequence of metasedimentary and metavolcanic rocks. The northern part of the batholith, about 40 by 120 km, contains a typical calc-alkalic plutonic assemblage. AFM and other variation diagrams for the Ben Ghnema batholith are virtually identical to those for the Sierra Nevada and Southern California batholiths. Furthermore, the Ben Ghnema batholith exhibits compositional variations from tonalite and granodiorite in the east to adamellite and granite in the west, identical to the west-to-east variation in the Sierra Nevada. The age of the Ben Ghnema batholith is 550 m.y. (Pan-African) by K-Ar and Rb-Sr methods. By analogy with the Sierra Nevada batholith, the Ben Ghnema batholith may have formed above a subduction zone carrying oceanic crust westward under a continental margin located in the northwestern Tibesti massif. This conclusion probably implies eastward extension of the West African craton across areas now occupied by the Ahoggar massif and the Murzuk basin. Events of Pan-African time in these areas were probably related to intracratonic reactivation. It is also possible that an oceanic basin extended eastward from the Tibesti massif to the western margin of the Arabian shield in Saudi Arabia, where Greenwood and others (1976) have postulated northeastward subduction of oceanic crust before and during Pan-African time.

Pan-African ages from a Tibesti Massif batholith, southern Libya

The northwestern portion of the Tibesti Massif in south-central Libya consists of a complex granitic batholith (the Ben Ghnema batholith) surrounded by medium- to low-grade metamorphic wall rocks. Rb-Sr whole-rock analyses of a suite of calc-alkaline rocks from the batholith give an age of about 560 m.y. and an initial 87Sr/ 86Sr ratio of about 0.706. This age is within the accepted range of the Pan-African orogeny which caused extensive plutonism throughout much of northwestern Africa. The relatively low initial ratio suggests that the magma was derived in the lower crust-upper mantle region and perhaps experienced some degree of mixing with older sialic material.

Genesis of the White Cloud and Related Pegmatites, South Platte Area, Jefferson County, Colorado

The writer mapped in detail the geology of an area of approximately 12 square miles, including South Platte, Colorado, to ascertain the geologic setting of large, concentrically zoned, rare-earth-bearing pegmatite bodies. These occur within granite of the northeastern portion of the Pikes Peak batholith. The outer batholithic zone of granite contains platy flow structure concentric with the concordant contact between the batholith and metasomatized metamorphic wall rocks, indicating emplacement of a mobile magma in the mesozone. Pegmatite and aplite dike emplacement, as well as development of tensional quartz veins with deuteric wall rock alteration and fluorite veins, mark the late stages of batholith formation. The rare-earth-bearing pegmatite bodies are lens-or pod-shaped, with quartz cores surrounded by four concentric but commonly incomplete zones. These zones are generally: (1) wall zones of graphic granite; (2) outer-intermediate zones of biotite, cyrtolite, and perthite; (3) middle-intermediate zones of giant perthite crystals; and (4) inner-intermediate zones of euhedral fluorite crystals. Compromise surfaces between some minerals are interpreted as evidence of simultaneous growth; crystallization is believed to have proceeded from the pegmatite walls inward following Bowen9s reaction series. Replacement at the White Cloud pegmatite consists of perthite by albite, fluorite by yttrofluorite, yttrofluorite by doverite, quartz and allanite by fluorite, biotite by hematite, quartz by sericite, and gadolinite by an unidentified fluocarbonate of yttrium with an X-ray structure similar to bastnae-site. Several rare-earth minerals have not been identified. Pegmatite emplacement is believed to have taken place by: (1) liquid segregation in a giant miarole, or (2) autointrusion, or (3) a combination thereof in the early stages of batholith crystallization. Paragenesis is interpreted in the light of recent laboratory findings of other workers supporting resurgent boiling and the action of a vapor phase as a transfer medium in replacement.

STRUCTURE AND PETROLOGY OF ENCHANTED ROCK BATHOLITH, LLANO AND GILLESPIE COUNTIES, TEXAS

Enchanted Rock batholith is one of several moderate-sized plutons that intruded a framework of tightly folded Precambrian metamorphic rocks in Middle Precambrian time. Tectonic features within the mass and structural attitude of wall rocks indicate the northern third is phacolithic. The remainder is both discordant and concordant batholithic, although the schists show a tendency to box the compass around the batholith. The phacolithic part occupies a synclinal trough plunging 35°–40° SE. The batholith consists of four concentric zones: (1) outer zone, medium to coarse leucogranite and granite; (2) intermediate zone, medium to coarse granite and quartz monzonite; (3) intermediate central zone, coarse quartz monzonite and alkalic granodiorite; and (4) the core, fine to medium leuco-quartz monzonite and leucogranite. Rapakivi are most abundant in zone 3. Seriate porphyritic texture prevails in all zones except the core where it is increasingly hiatal. Chilled border rocks are 10–20 feet wide and consist of leuco-quartz monzonite and quartz monzonite. Apophyses of the batholith in metamorphic wall rocks are leuco-quartz monzonite and leucogranite. Hiatal porphyritic texture prevails in chilled border and apophyses rocks. Internal structures of the batholith and its relation to country-rock structures indicate intrusion during the late, most severe stages of regional compression. Tensile stresses along axial planes of folds exerted structural control and guided the rise of the magma, as indicated by the pear-shaped outline of the batholith. The most probable mechanism of magma generation is selective fusion (melting) of random rock material. This occurred at depth in down-buckled zones below the present position of the batholith. Concentration of the proportions of normative constituents of individual rock zones is strikingly grouped about a composition having a 1:1:1 ratio of albite: orthoclase: quartz. Mineralogically there are systematic gradations in proportions of plagioclase, microcline, and quartz between rock zones suggesting that unmixing of sodic plagioclase from microcline was a dominant process. Outline shape of rock zones, textural relations between plagioclase and microcline crystals, and maxima-minima concentrations of unmixed sodic plagioclase suggest migration of part of the unmixed material within the batholith during the later stages of its structural and intrusive history. Field and chemical evidence indicate that probably not more than 5 per cent of the granites of the batholith are of replacement origin. Failure of roof rocks with partial escape of volatiles, cessation of regional tectonism, and depletion of the magma reservoir in the late stages of intrusion probably are responsible for finer grain of the core. Age determinations by the “Larsen Method” give an average age for the batholith of 815 million years.

Mt. Waldo batholith and associated igneous rocks, Waldo County, Maine

The central part of the Bucksport, Maine, topographic map area is underlain by a medium-textured, porphyritic, biotite granite, termed here the Mt. Waldo batholith. The age of the granite has not been determined but is probably either Devonian or Carboniferous. The pluton is roughly circular in plan, with a diameter of approximately 10 miles. Planar flow structures in the granite tending to curve parallel to the margins of the intrusion about the northern end stand steep to vertical. Within the intrusion, the planar flow structure averages about north in strike and vertical in dip. Linear structure is not developed. On the north, northeast, and south, the older metamorphic wall rocks, probably Paleozoic, are crosscut by the granite; elsewhere, the structure of the wall rocks tends to conform to the outlines of the intrusion. Close to the intrusion, the axes of the minor folds in the wall rocks are steeply inclined or vertical; farther from the margins, the pitch of minor folds decreases. The overlying rocks were perhaps pushed bodily upward by the intrusion. Intrusives older than the granites of the Mt. Waldo batholith are both granitic and more basic. Both groups are of undetermined age. The earlier gneissic granites are foliated and locally show linear structure consisting of biotite streaking on the foliation planes. This type outcrops in small masses. Pegmatite and aplite dikes are present, some older and others younger than the batholith granites. Diabase dikes are the latest intrusions in the area.