Quartz Megacrysts Research Articles

Supercritical fluids as recorded in quartz megacrysts of the Late Jurassic porphyritic granitic dyke in the giant Dongping gold deposit, Northern China

The giant Dongping gold deposit, hosted in Devonian alkaline complex, is the first deposit of this kind discovered in China. In this deposit, a newly found porphyritic granitic dyke that contains abundant quartz megacrysts was studied. The dyke occurs along a NE-SW striking shear zone and has gold mineralization within the shear zone. Orthoclase from porphyritic granitic dyke has good plateau ages of 144.3 ∼ 148.2 Ma and isochronal ages of 145.4 ∼ 146.7 ± 1.53 Ma, similar to zircon U-Pb ages of a previous study for the porphyritic dyke. This indicates that the porphyritic granitic dyke is coeval with the adjacent Late Jurassic Shangshuiquan K-feldspar-rich granitic pluton. Plentiful primary fluid inclusions in those quartz megacrysts have homogenization temperatures (300 °C to 400 °C) similar to those in Au-bearing quartz veins of the Dongping deposit, with lower salinities of 5.0–19.0 wt%NaCl equiv. A small number of melt inclusions which can be classified as melt-fluid inclusions were seen in quartz megacrysts together with fluid inclusions. It is suggested that the initial fluids related to the formation of porphyritic granitic dyke could be H2O-silicate supercritical fluids. We propose that H2O-silicate supercritical fluids containing Au were originated from Late Jurassic K-feldspar-rich granitic magmas and might further leach Au from country rocks of alkaline complex. These fluids, which migrated along NE-SW shear zone that controlled the porphyritic granitic dyke, replaced alkaline complex, formed granitic dyke by replacement related to granitization, and, later, Au-bearing quartz veins.

A mineralogia acessória do Amazonita Pegmatito Serra Branca: classificação de um pegmatito NYF na Província Borborema, Nordeste do Brasil

O campo pegmatítico de Vieirópolis é caracterizado pela ocorrência de pegmatitos com amazonita e/ou berilo. Esse campo contém pegmatitos do tipo NYF (Nb-Y-F) inseridos no Domínio Rio Grande do Norte (DRGN), fora do contexto da Província Pegmatítica do Seridó (PPS). O principal pegmatito desse campo, o Amazonita Pegmatito Serra Branca, destaca-se pela mineralização de megacristais de amazonita e por sua exploração como rocha ornamental de elevado valor econômico. Para caracterizar a paragênese e a química mineral das fases acessórias do Amazonita Pegmatito Serra Branca, além de elucidar a classificação e a natureza petrogenética desse pegmatito, foram utilizados dados de campo, petrográficos, difração de raios X e química mineral. O Amazonita Pegmatito Serra Branca ocorre como dique tabular com aproximadamente 3 m de espessura, de direção NW-SE e caimento 45° WSW, caracterizado por apresentar uma zona composta por megacristais de amazonita e quartzo, e outra, por albita e quartzo sacaroidais. Apresenta vasta mineralogia acessória que compreende anglesita, biotita, bismutita, cerussita, columbita-(Mn), espessartina, helvina, fenaquita, fluorita, hematita, ilita, ilmenita, magnetita, montmorilonita, muscovita, piromorfita, pirocloro, rutilo e zircão. Destacando-se, também, atípicas mineralizações de sulfetos e a presença de beríliossilicatos reportados pioneiramente no contexto geológico da Província Borborema (PB). A paragênese mineral e a química dos minerais acessórios do Amazonita Pegmatito Serra Branca o classificam como pertence à família NYF da classe dos elementos raros, tipo da gadolinita, possivelmente associado a um magmatismo do tipo I.

Immiscible-melt inclusions in corundum megacrysts: Microanalyses and geological implications

Abstract Controversies on the origin of zircon, corundum, titanomagnetite, and quartz megacrysts in alkali basalts mostly reflect the lack of direct evidence of a “melt reservoir” required for their formation. Various mineral megacrysts are carried up by Cenozoic (mostly younger than 25 Ma) alkali basalts that extend more than 4000 km along eastern China. Here we report unusual inclusions in corundum megacrysts from Changle, and we attribute their origin to the existence of a FeO*-SiO2-Al2O3-ZrO2-rich melt. The inclusions, analyzed using electron microprobe and Raman microscopy, may be divided into two types. Type I inclusions are dominated by glassy materials, may exhibit a dark part in backscattered eletron (BSE) images composed of quartz, corundum, and an amorphous substance (AS-1), and a bright part in BSE images composed of baddeleyite and a second distinct amorphous substance (AS-2). Compared with AS-1, AS-2 has higher concentrations of ZrO2 and FeO* but lower concentrations of Al2O3 and SiO2. We argue that the formation temperature of Type I inclusions is ~1200 °C, and the generation of their bright and dark parts in BSE images may be attributed to the coexistence of immiscible melts. Type II inclusions are composed of zircon, quartz, and an amorphous substance (AS-3). Both types of inclusions might be derived from a similar parent melt, which is FeO*-SiO2-Al2O3-ZrO2-rich. New secondary ion mass spectroscopy (SIMS) in situ U-Pb ages of 18 Ma and 13–14 Ma for zircon inclusions suggest that the corundum megacrysts, occurring in basaltic host rocks distributed along the middle segment of the north and south-trending Tanlu fault zone, formed from precursor residual magmas related to underplating basalts stalled at the crust-mantle boundary, and were brought to the surface by entrainment in later basalts. This study provides new insights into the genesis of the corundum-related megacryst suite.

Magmatic-hydrothermal transition of Mo-W-mineralized granite-pegmatite-greisen system recorded by trace elements in quartz: Krupka district, Eastern Krušné hory/Erzgebirge

Magmatic-hydrothermal transition of highly evolved granitic melts enriched in volatile and incompatible elements can involve disequilibrium intermediate products between aluminosilicate melt and hydrothermal fluid (hydrosilicate liquids) with high ore-forming potential. We investigate evolution from Li-F-rich granites through pegmatites, greisens and quartz veins in a composite stock at Knöttel in the Krupka district (eastern Krušné hory/Erzgebirge mountain range) by monitoring trace-element variations in quartz. Interelemental correlations reveal Si4+ ↔ Li+Al3+ and likely Si4+ ↔ H+Al3+ to be the most important substitution mechanisms. Variations in Ti vs. Li, Be and Al define distinct evolutionary trends: (i) magmatic, high-Li/Ti or Al/Ti trend (granites – aplites – K-feldspar pegmatite); (ii) transitional, medium-Li/Ti trend (quartz megacrysts and pegmatite lenses in granite – quartz-protolithionite pegmatite); (iii) hydrothermal, low-Li/Ti or Al/Ti trend (stockwork of coarse-grained hydrothermal quartzites and quartz veins, quartz replacement in greisens). The medium-Li/Ti trend represents hydrosilicate liquid, an H2O- and SiO2-rich medium with low density and effective viscosity that was probably formed during disequilibrium crystallization in front of rapidly propagating solidification front. Thermal evolution of the magmatic-hydrothermal system was monitored by Ti-in-quartz thermometry. Calculated rutile activity of the granites is very low (0.3–0.05) but it increases (up to 1, that is, saturation) towards pegmatites and hydrothermal veins. Magmatic crystallization (granites and aplites) is constrained to 700–580 °C, pegmatite and bulk greisenization stage occurred at 600–500 °C, followed by a late hydrothermal stage associated with the formation of distal quartz veins at 500–390 °C. The granite-pegmatite systems at Knöttel and in Erzgebirge in general reach extremely high Al, Li, Rb and Ge and low Ti concentrations in quartz in comparison with igneous rocks worldwide. The Knöttel stock belongs to P-poor A-type granites in the Erzgebirge and is linked with them by similar evolutionary trends in quartz trace elements. Furthermore, the Knöttel system exhibits Be enrichment in quartz (probably related to high contents of F) and this appears to be typical feature of Mo-W-mineralized systems in general. The hydrothermal phase with Mo-W mineralization at Knöttel is closely related to the magmatic phase of parental granite body, which seems to be an important condition for Mo-W-mineralization formation.

QUARTZ MEGACRYSTS IN GREECE: MINERALOGY AND ENVIRONMENT OF FORMATION

Quartz megacrysts in Greece are systematically sampled, described and classified with respect to their morphology, solid inclusion mineralogy, and geological conditions of formation. Quartz deposition took place due to reduction in silica solubility in the hydrothermal fluids in favourable geological environments such as: alpine-type fissures close to major detachment faults, skarns and quartz veins crosscutting and generally related to granitoids and, finally, epithermally altered volcanic rocks of Tertiary age. The varieties of coloured crystals (amethyst, smoky quartz, morion, green quartz, rock crystal), the twinning of the crystals, the mineralogy of solid phases included in quartz (rutile, chlorite, sericite, feldspars) as well as the types and forms of the crystals (Tessin habit, Muzo habit, faden quartz, sceptre, window quartz, interrupted crystals, double-terminated crystals, phantom quartz, gwindel quartz, etc.) give important information on the growth mechanisms and the physico-chemical conditions during quartz formation. The quartz crystals found in several localities are gemmy and their potential for use as gemstones should be evaluated.

Petrogenesis of granitoids in the Dewulu skarn copper deposit: implications for the evolution of the Paleotethys ocean and mineralization in Western Qinling, China

The Tethyan tectonic domain hosts numerous world-class mineral deposits. Among these, the Dewulu skarn copper deposit in Western Qinling, China belongs to the Paleotethys ore belt. The skarn and orebodies here occur as stratoids or lenses at the contact between the Triassic Dewulu intrusive complex and Permian marine clastic and carbonates. Alteration minerals include prograde skarns (garnet, diopside, wollastonite), plagioclase, hornblende, actinolite, tremolite, epidote, chlorite, calcite, quartz and sericite. The main ore types include early disseminated skarn-type replacement orebodies and late-stage quartz-sulfide veins. Chalcopyrite is the major ore mineral, along with pyrite, bornite and sphalerite. The Dewulu intrusive complex comprises quartz diorite, quartz diorite porphyry and dioritic mafic microgranular enclaves (MME). The MMEs are spheroidal in shape, and have igneous mineral assemblages, acicular apatites, complex oscillatory zoned plagioclase and quartz megacrysts surrounded by mafic minerals. The MMEs are metaluminous and calc-alkaline to high-K calc-alkaline, and possess relatively high Ni, Cr and MgO contents and Mg# values. They display sub-parallel patterns in trace element spider diagrams and rare earth element (REE) plots. They are also characterized by the enrichment of Rb, U and Th, depletion of Ba, Sr, Nb and Ta and negative Eu anomaly. Zircon LA-ICP-MS U–Pb dating of the dioritic MME yields an age of 247.0±2.2Ma, coeval with the host quartz diorite, quartz diorite porphyry and ore-related sericite 40Ar/39Ar plateau ages within analytical uncertainties. Oxygen fugacity estimated from trace element compositions of zircons from the dioritic MME shows FMQ±3.3. The zircons have negative εHf(t) values in a range of −8.0 to −3.3, corresponding to two-stage model ages ranging from 1.48 to 1.78Ga. The integrated data from petrology, geochronology and bulk geochemistry suggest that the Early Triassic granitoids associated with Cu skarn mineralization at Dewulu were products of arc magmatism and involved magma mixing in an active continental margin setting. The magma was sourced through partial melting of enriched sub-continental lithospheric mantle that had been previously modified by slab-derived melt during the continuous northward subduction of the Paleotethys oceanic slab.

The nature of “quartz eyes” hosted by dykes associated with Au-Bi-As-Cu, Mo-Cu, and Base-metal-Au-Ag mineral occurrences in the Mountain Freegold region (Dawson Range), Yukon, Canada

*Corresponding author Various origins have been assigned to rounded to subrounded and elliptical quartz megacrysts (“quartz eyes”) in dyke rocks associated with mineral deposits/occurrences worldwide. An exact interpretation of their nature is likely to tightly constrain the petrogenesis of the host rocks, and by association may be critical in evaluating genetic models for spatially associated ore minerals. ChromaSEM-CL imaging and electron probe microanalysis (EPMA) of “quartz eyes” within porphyry dykes associated with Au–Bi–Cu–As, Mo–Cu, and base-metal–Au–Ag mineral occurrences across the Northern Freegold Resources (NFR) property in the Dawson Range of Yukon Territory, Canada, reveals that the cathodoluminescence (CL) response of quartz is a function of its trace-element abundance(s). Bright blue luminescent growth zones are in most cases richer in Fe (up to 8839 ppm) and Ti (up to 229 ppm) relative to CL dark growth zones, with up to 41 times lower concentration of these elements. Assuming a TiO 2 = 1, the Ti-poor dull cores consistently recorded lower temperatures (mostly < 600 °C) compared to Ti-rich brighter blue rims (up to 860 °C). This suggests either overgrowth on xenocrystic cores or an increase in crystallization temperature. The temperature rise likely reflects magma mixing, and is therefore consistent with the phenocryst/phenoclasts having formed in a magma chamber rather than by secondary processes. Also, the great variability in composition and temperature of crystallization and/or reequilibration of brighter blue growth zones of two quartz crystals (660 °C and 855 °C) from a single sample suggests that multiple episodes of magma mixing and incremental growth of parental magma chambers occurred. Some “quartz eyes” are overprinted by variably oriented, bifurcating, and anastomosing fluid migration trails (“splatter and cobweb textu res”) of red to reddish-brown CL quartz that is in most cases of low-temperature origin, and trace-elements poor, thus implying interaction of deuteric fluids with quartz phenocrysts/phenoclasts. The presence of “quartz eye” crystals with broken and angular blue cores, overgrown by oscillatory-zoned rims in which the zoning pattern does not correspond with the crystal boundaries, further suggests that some quartz crystals had been explosively fragmented (phenocrysts) and are now hosted in a recrystallized tuffisitic groundmass. The volatile exsolution that likely accompanied both magma mixing and decompression (as suggested by dendritic quartz, fine-grained recrystallized tuffisitic groundmass, and corroded quartz grains) was probably an important process that could have favoured the ore formation.

Origin of lamprophyres by the mixing of basic and alkaline melts in magma chamber in Beiya area, western Yunnan, China

Lamprophyres consisting mainly of diopside, phlogopite and K-feldspar formed in the early Tertiary around 60 Ma in the Beiya area and are characterized by low SiO 2 ± 46–50 wt.%), Rb (31–45 ppm) and Sr (225–262 ppm), high Al 2O 3, (11.2–13.1 wt.%), CaO (8.0–8.7 wt.%), MgO (11.5–12.1 wt.%), K 2O(4.9–5.5 wt.%), TiO 2 (2.9–3.3 wt.%) and REE (174–177 ppm), and compatible elements (e.g. Sc, Cr and Ni) and HSF elements (e.g. Th, U, Zr, Nb, Ta, Ti and Y), and low 143Nd/ 144Nd 0.512372–0.512536, middle 87Sr/ 86Sr 0.707322–0.707395, middle 206Pb/ 204Pb 18.50–18.59, 207Pb/ 204Pb 15.60–15.65 and 208Pb/ 204Pb 38.75–38.8. These rocks developed peculiar quartz megacrysts with poly-layer reaction zones, melt inclusions, and partial melted K-feldspar and plagioclase inclusions, and plastic shapes. Important features of these rocks include: (1) hybrid composition of elements, (2) abrupt increase of SiO 2 content of the melt, recorded by zoned diopside, (3) development of sanidine and aegirine-augite reaction zones, (4) alkaline melt and partial melted K-feldspar and plagioclase inclusions, (5) deformed quartz inclusions associated with quartz megacrysts, (6) the presence of quartz megacrysts in plastic shape with their parent melts, (7) the occurrence of olivine, high-MgO ilmenite and spinel inclusions within earlier formed diopside, phlogopite and magnetite. Median 87Sr/ 86Sr values between Tertiary alkaline porphyries in the Beiya area and the western Yunnan and Tertiary basalt in the western Yunnan indicate that the Beiya lamprophyre melts were derivative and resulted from the mixing between basic melts that were related to the partial melting of phenocrysts of spinel iherzolite from a mantle source. The alkaline melts originated from partial melting along the Jinshajiang subduction ductile shear zone at the contact between the buried Palaeo-Tethyan oceanic lithosphere and the upper mantle lithosphere. The alkaline melts are composed of 65% sanidine (Or 70Ab 28An 2) and 35% SiO 2. The melt mixing occurred in magma chambers in the middle-shallow crust at 8–10 km before the derivative lamprophyre melts intruded into the shallow cover in Beiya area. This mixing of basic and alkaline melts might represent a general process for the formation of lamprophyre in the western Yunnan.

The origin of rapakivi texture by sub-isothermal decompression

The rapakivi texture (plagioclase-mantled K-feldspar ovoids, rounded quartz megacrysts and euhedral plagioclase megacrysts in a more fine-grained granitic matrix) has been studied in five Proterozoic (1.64–1.55 Ga) anorogenic rapakivi granite batholiths in the Fennoscandian Shield with emphasis on mineral stability and inherited cores. K-feldspar ovoids and rounded quartz megacrysts with deep embayments consist principally of a core zone and an outer melted and recrystallized zone. When the K-feldspar ovoids are mantled by plagioclase, the outermost part of the mantle is homogeneous, but towards the K-feldspar, a skeletal texture develops. Intensive parameters obtained from different textural positions show that mineral inclusions in cores of the K-feldspar ovoids and quartz megacrysts were formed at low T (∼680–720°C) and high P (5–6 kbar) conditions, while inclusions in the periphery of ovoids and the plagioclase mantles display high T (∼780°C) and intermediate to low P (3.5–1 kbar). The lowest P is comparable to that during solidification of the matrix. The total water equivalent of the volatile content in the magma has been calculated as ∼2.5%. The amount of solids in the magma at ∼1 kbar has been estimated as ∼40%. Theoretical calculations and experimental data for mineral stabilities in granitic systems suggest that the texture formed when a crystal-saturated (Kfsp+Qtz+Pl ∼60%) and volatile-undersaturated A-type granite magma was transported under approximately constant temperature (760–780°C) from the lower-middle crust (5–6 kbar) to upper crustal levels. According to phase stabilities in the eutectoid granite system, quartz and K-feldspar were resorbed but plagioclase remained stable and precipitated during this sub-isothermal rise of magma. Textural (presence of disequilibrium textures) and mineralogical (presence of different mineral assemblages, including relics) evidence of a sub-isothermal rise of the rapakivi magmas is better preserved in subvolcanic and contact varieties of rapakivi granites than in the more deep-seated rapakivi granites formed by slow cooling.

The Role of Diffusion-controlled Oscillatory Nucleation in the Formation of Line Rock in Pegmatite-Aplite Dikes

The George Ashley Block (GAB), located in the Pala Pegmatite District, San Diego County, California, is a composite pegmatite–aplite dike of 8 m thickness displaying striking mineralogical layering in the aplite portion of the dike, referred to as line rock. Rhythmic layering is characterized by garnet-rich bands alternating with albite–quartz–muscovite-rich bands. Cumulus textures are notably absent from the layered portion of the dike. Elongated quartz megacrysts are oriented perpendicular to the garnet-rich layers and poikilitically include garnet, albite, and muscovite. Calculated crystal-free magma viscosity with 3% H2O is 106.2 Pa s and the calculated settling velocity for garnet is 0.51 cm/year. Conductive cooling calculations based on emplacement of a 650°C dike into 150°C fractured gabbroic country rock at 1.5 kbar, and accounting for latent heat of crystallization, demonstrate that the line rock portion of the dike cools to 550°C in about 1 year. Crystal size distribution studies also suggest very rapid nucleation and crystallization. Diffusion-controlled gel crystallization experiments yield textures virtually identical to those observed in the layered aplite, including rhythmic banding, colloform layering, and band discontinuities. Thus, observed textures and calculated magmatic parameters suggest that mineralogical layering in the GAB results from an in situ diffusion-controlled process of oscillatory nucleation and crystallization. We propose that any event that promotes strong undercooling has the potential to initiate rapid heterogeneous nucleation and oscillatory crystal growth, leading to the development of a layer of excluded components in front of the crystallization front, and the formation of line rock.

Charnoquitóides do maciço de Várzea Alegre: um novo exemplo do magmatismo Ca-alcalino de alto K no arco magmático do Espírito Santo

CHARNOCKITIC ROCKS OF THE VA RZEA ALEGRE MASSIF: A NEW EXAMPLE OF THE HIGH K CA-ALKALIC MAGMATISM IN THE MAGMATIC ARC OF THE ESPIRITO SANTO The Varzea Alegre intrusive massif is situated in the central portion of the Espirito Santo State. It is an example of the late to post tectonic magmatism related to the onset of a magmatic are developed during the late-Brasiliano episode. It comprises an inner domain of gabbros, diorites, quartz-diorites and granites completely surrounded by a large ring of charnockitic rocks. The lithotypes found in that ring were chemically classified as opdalites, jotunites, quartz mangerites and enderbites (opx-quartz diorites). They show aporphyritic texture with feldspars and quartz megacrysts set in a medium to coarse grained matrix. The geochemical characteristics of these rocks are typical of a calc-alkalic magmatism enriched in LIL and HFS elements (mainly Ba, K, REE, Zr, P and Nb). Using compatible versus incompatible elements variation diagrams it is possible to sugest that fractional crystalization and magma mixing were the principal mechanisms in the evolution of the suite. The high contents in such incompatible elements point towards a mantelic contribution associated with crustal contamination for the genesis of the rocks.

Orbicular volcanic rocks of Cerro Panizos: Their origin and implications for orb formation

Orbs, which are a rare occurrence in granitoid rocks, are present in a 5-m-thick stratum within the Cerro Panizos Ignimbrite and in two post-ignimbrite lava flows. Orbs in volcanic host rocks are extremely rare and have not been described in detail previously. The orbs consist of two to five crystalline rings surrounding a xenolithic or orthopyroxene core. The rings alternate between bands of large, radially oriented plagioclase and orthopyroxene crystals and bands of small, tangentially or radially oriented biotite and ilmenite crystals. The ignimbrite orbs are associated with two types of pumice: (1) a coarsely porphyritic biotite-quartz-plagioclase dacite with 35%-40% crystals found throughout the ignimbrite and (2) a finely porphyritic biotite-plagioclase quartz dacite with 75%-80% crystals found only in association with the orbs. The major- and trace-element and isotopic compositions of the two pumice types are identical. Bronzite and quartz megacrysts are also found with the ignimbrite orbs and rarely occur in the overlying sequence. Plagioclase and orthopyroxene compositions in orb-associated rocks exhibit large variations. The orb inner rings and finely porphyritic pumice have the most mafic compositions (An81-92 and En60-77), coarsely porphyritic pumice and lavas have the most felsic compositions (mostly An45-63 and En42-50), and the outer rings of the orbs span the entire gap between the two groups (An67-77 and En50-66). The orbs formed in a water-rich cupola along the roof of the magma body, where the magma was superheated and most crystals were resorbed. Pressure release related to eruption caused exsolution of water, leading to large degrees of undercooling. Orbs formed rapidly around the few available nuclei. As limited mixing with the surrounding coarsely porphyritic magma occurred, heterogeneous nucleation in the supercooled magma began, forming the abundant small crystals seen in the finely porphyritic pumice. Eruption of the orbicular dacite occurred when a ring vent conduit tapped the magma in the cupola. Similar processes may form orbs in plutonic rocks, with pressure release related to either eruption or intrusion to higher levels in the crust.

Origin and metamorphism of peperite and associated rocks in the Devonian Elwell Formation, northern Sierra Nevada, California

The Late Devonian Elwell Formation is part of a late Paleozoic island-arc assemblage exposed in the northern Sierra Nevada, California. In the 10-km interval from Mount Elwell (Plumas County) to Packer Lake (Sierra County), it consists largely of numerous andesitic sills interleaved with phosphatic radiolarian chert and turbidity-current and debris-flow deposits. Peperite, formed by interaction of intrusive andesitic magma and still-moist, incompletely lithified “chert” not far beneath the sea floor, occurs sparingly. The whole assemblage was thoroughly recrystallized (greenschist-facies regional metamorphism) during the Late Jurassic Nevadan orogeny. The phosphatic chert is composed of white, fluorapatite-rich lenses and nodules enclosed in a black matrix consisting of microgranular quartz and carbonaceous material (probably graphite). Radiolaria (family Entactiniidae) are so abundant that the chert is thought to have originated as radiolarian ooze. The phosphatic lenses and nodules probably formed by direct inorganic precipitation of fluorapatite from pore water within decaying radiolarians at the sediment-water interface. The andesitic sills are generally aphyric and aphanitic; original textures varied from hyalopilitic in sill interiors to pilotaxitic at sill margins. The andesite was charged with plagioclase (now albite) microlites, and quench morphologies occur where sills intruded the wettest “chert.” Relatively thick sills commonly display columnar structure, whereas a few originally more viscous sills are flow laminated. The top of a sill emplaced essentially at the sediment-water interface is pillowed. Many sills bear xenoliths of phosphatic chert, and scarce quartz megacrysts (evidently derived from the underlying, dacitic Sierra Buttes Formation) appear nearly everywhere. Concentrations of major-element oxides and immobile trace elements indicate that the andesite originated as part of the tholeiitic rock association of island arcs; rare-earth–abundance patterns are particularly diagnostic of this paleotectonic setting. Most peperite has poorly sorted (coarse ash to coarse block) andesitic fragments uniformly distributed through a voluminous black chert matrix. Where invaded “chert” retained the most pore water, quenched andesitic magma formed irregularly shaped lapilli and small pillows that initially exhibited ductile behavior. Rapid cooling below the softening point of glass resulted in brittle fracture of the lapilli during dispersal through the “chert” by steam explosions. Steam explosions also incorporated into the peperite rigid clasts of locally dewatered chert, and shattered already-formed phosphatic nodules. Where the invaded “chert” was better lithified, freezing sill margins simply granulated quietly in situ by thermal-contraction cracking of microlitic glass; fluid “chert” injected widely spaced contraction joints deep within the sills. In all cases, there was sufficient heated connate water to cause palagonitization and chloritization of andesitic fragments. Greenschist-facies metamorphism produced two mineral assemblages in the interiors of andesitic sills, the second perhaps reflecting locally higher Pco 2 /Ph 2 o during metamorphism: (1) albite + quartz + epidote + actinolite + chlorite + Ti-rich mineral ± calcite ± stilpnomelane ± white mica, and (2) albite + quartz + chlorite + Ti-rich mineral + calcite ± stilpnomelane ± white mica. Epidote-rich specks occurring sporadically in the sills represent metasomatism on a scale of millimetres to centimetres; such specks are mineralogically zoned in andesitic fragments in intensely recrystallized peperite. Additional examples of local meta-somatic activity include syntectonic crystallization of calcite rhombohedra across original vesicle walls in a sill. Cleavage developed locally during regional metamorphism is parallel to axial planes of closely appressed folds that deformed andesitic fragments in peperite and flow laminae in sills.

Inverted high-temperature quartz

Fifty-two samples of inverted high-temperature quartz from volcanic rocks were investigated by Guinier-Jago powder diffractometry and differential scanning calorimetry (DSC). Quartz megacrysts from Clear Lake and Cinder Cone, California show a variability of ≃2.5 ° K in their α-β transition temperature (T α-β). Quartz phenocrysts and quartz from crystalline rocks give a range of 0.5 ° K in T α-β. Neutron activation analysis of single crystals demonstrates that Al is the principal impurity (17–380 ppm). Its concentration is inversely correlated with T α-β. A very small variation was found in the a and c lattice parameters among the specimens of volcanic quartz studied. This variation does not correlate with Al content or transition temperature. Mean values at 22 ° C (a=4.1934±0.0004 A, c=5.4046±0.0006 A) are similar to those of quartz grown at low temperatures. Enthalpy of the α-β transition (ΔH α-β), obtained over 9.0 ° from DSC runs, is dependent upon sample grain size and for a crushed powder with zero hysteresis (T α-β on heating=T α-β on cooling) is 92.0 ±1.4 cal/mol. In contrast, a single piece of quartz requires ΔH α-β be 107.7±1.4 cal/mol and has a T α-β hysteresis of 1.1 ° K. Regression of published data provides equations for the variation of the molar volume (cc/mol) of quartz with v. These equations imply a ΔV α-β of 0.205±0.031 cc/- mol. Expressions are also provided for the temperature dependence of the thermal coefficient of expansion, α, the compressibility, β, and (∂/gb/∂T)p (which is identically -(∂α/∂P) T ). DSC heat capacity measurements over the range 400 to 900 ° K were fitted to extended Maier-Kelley type expressions to give: $$\begin{gathered} C_P = 10.31 + 9.116 \times 10^{ - 3} T - \frac{{1.812 \times 10^5 }}{{T^2 }} \hfill \\ - {\text{5}}{\text{.630}} \times 10^{ - 2} {\text{ }}\frac{T}{{(T - 848)}} - 0.3553\frac{T}{{(T - 848)^2 }} \hfill \\ - 0.9011\frac{T}{{\left( {T - 848} \right)^3 }} \hfill \\ (400{\text{ to 842}}^ \circ {\text{K), and}} \hfill \\ C_P = - 318.8 + 0.2532T \hfill \\ {\text{ + }}\frac{{8.687 \times 10^7 }}{{T^2 }} + 0.1603\frac{T}{{\left( {T - 848} \right)^4 }} \hfill \\ \end{gathered} $$ (851 to 900 ° K), which together with the values of ΔH α−β measured over the range 842–851° K give 7875.3 cal/mol for H900-H400. The behavior of α, β, and C p as a function of T emphasizes that structural changes which occur at the α−β transition do so over a broad temperature interval.

Dolomitization and the mineralization of the Marl Slate (N. E. England)

The Marl Slate, the English equivalent of the Kupferschiefer, has been studied with particular reference to the relationships between dolomitization and the origin of the metal sulphides. Dolomite occurs as: 1) discontinuous lenses of ferroan dolomicrite, 2) micronodules of finely crystalline dolospar in association with length-slow chalcedony and 3) discrete laminae of ferroan or non-ferroan dolospar. The ferroan dolomicrite has excess CaCO3, and is more abundant in the lower, sapropelic facies of the Marl Slate. It is considered to have formed by the penecontemporaneous alteration of calcium carbonate under hypersaline conditions. Small micronodules (typically about 0.3 mm in diameter) are also more abundant in the sapropelic Marl Slate. These frequently contain cores of length-slow chalcedony (quartzine) fibres and sometimes quartz megacrysts. Textural observations clearly indicate that this silica is of authigenic origin and the dolomite/chalcedony micronodules are interpreted as diagenetic replacements of a calcium sulphate mineral such as anhydrite. The discrete laminae of finely crystalline dolospar are often inter-laminated with calcite in the upper part of the Marl Slate. This dolomite is also calcium rich and represents a replacement, possibly of anhydrite, during a later phase of diagenesis. Metal sulphides occur in two distinct forms: as disseminated framboidal pyrite and as discrete lenses of pyrite, chalcopyrite, galena, sphalerite and rarer sulphides. The framboidal pyrite originated during early diagenesis by reaction of sulphide, produced by reduction of sulphate by organic material and micro-organisms, with iron also released in the reducing environment. The sulphide lenses are often in intimate association with dolospar, length-slow chalcedony and authigenic quartz megacrysts. This indicates that the lenses were produced during diagenesis by the reduction and replacement of calcium sulphate (anhydrite). Various sources, such as co-precipitation with dolomite precursors and the underlying Yellow Sands, may have supplied metals which were mobilized and transported by connate brines as diagenesis progressed.