Biotite Quartz Monzonite Research Articles

Geochemical and REE mineralogical characteristics of the Zhaibei Granite in Jiangxi Province, southern China, and a model for the genesis of ion-adsorption REE deposits

The Zhaibei Granite in Jiangxi Province, southern China, hosts an ion-adsorption light rare earth element (LREE) deposit. Recently, heavy REE (HREE) ores have also been reported from weathered crusts of the granitic rocks. In this study, petrological, geochemical, and mineralogical characteristics of the Zhaibei pluton were analysed to establish the genesis of this REE deposit. The pluton contains coarse-grained biotite syenogranite and minor hornblende–biotite–quartz monzonite in its central zone, medium-grained biotite syenogranite in its transitional zone, and fine-grained muscovitic alkali-feldspar granite and porphyritic muscovitic biotite–alkali-feldspar granite together with monzogranite porphyry intrusions in its marginal zone. The REE minerals include titanite, allanite, monazite, bastnasite, and thorite-(Y) in the central zone, allanite, monazite, xenotime, bastnasite, and thorite-(Y) in the transitional zone, and synchysite-(Y), thorite-(Y), and xenotime in the marginal zone. The SiO2 content increases from 62 to 77 wt% from the centre to the margin, whereas the Al2O3, TiO2, MgO, FeO, and CaO contents decrease. In addition, the size of the negative Eu anomaly and HREE content increase, accompanied by decreases in Co, Zr, Hf, Sr, and Ba and increases in Rb, Cs, Nb, and Sn. These observations indicate that the Zhaibei Granite might have been formed by crystallisation differentiation of magma. The granitic pluton was also influenced by reactions with late-magmatic F-, CO2–, and REE-rich fluids that altered magmatic minerals and crystallised as REE-fluorocarbonates and thorite. The hydrothermal REE minerals constitute ∼ 40% of the total REEs in the bedrock and are the major supplier of ion-exchangeable REEs in the ores of the Zhaibei deposit. LREE-rich ores may have been derived from the LREE-rich granitic rocks, which contain bastnasite, titanite, and allanite, whereas HREE-rich ores were sourced mainly from the HREE-rich granites, which contain synchysite-(Y) and thorite-(Y). Furthermore, REE ores with enrichment in both HREEs and LREEs were probably sourced from the monzogranite porphyry bedrock, which contains bastnasite-(Y), bastnasite, allanite, and thorite-(Y).

Wumuite (KAl<sub>0.33</sub>W<sub>2.67</sub>O<sub>9</sub>) – a new mineral with an HTB-type structure from the Panzhihua–Xichang region in China

Abstract. Wumuite, ideally KAl0.33W2.67O9, is a new mineral species found in the Neoproterozoic Sinian light-weathered biotite–quartz monzonite in the southern part of the Panzhihua–Xichang region (Nanyang village: 26∘46′8.21′′ N, 101∘27′13.86′′ E), China. It is associated with quartz, orthoclase, albite, biotite, hornblende, kaolinite, ilmenite, goethite, hematite, zircon, zoisite, tourmaline, monazite-(Ce), allanite-(Ce), scheelite, tellurite, tewite, and an unidentified, potentially new mineral corresponding to WO3. Wumuite occurs as light green hexagonal tabular crystals, is up to 0.3 mm in diameter, and has a vitreous to adamantine luster and a white streak; i.e., it is transparent. The mineral is brittle with good cleavage parallel to {101¯0} and {0001}. It has a Mohs hardness of about 5–6 and a calculated density of 6.52 g cm−3. Electron microprobe analyses yielded the following (in wt % – average of 10 spot analyses of one sample): K2O 5.55, WO3 91.16, TeO2 0.59, and Al2O3 2.52, with a total of 99.82. The empirical formula for wumuite calculated on the basis of Oapfu=9 is K0.80(W2.68Al0.34Te0.03)∑3.05O9, ideally K(W2.67Al0.33)∑3O9 or KAl0.33W2.67O9. The strongest four diffraction lines [d Å (I) (hkl)] are 6.261(36)(010), 3.727(30)(001), 3.161(100)(020), and 2.413(40)(021). Wumuite is hexagonal, in space group P6/mmm, with a=7.2952(5) Å, c=3.7711(3) Å, V=173.81(2) Å3, and Z=1. The crystal structure was solved and refined to a reliability factor of R1[F2>4σ(F2)]=0.025 (wR2=0.072) based on 164 unique reflections (777 measured reflections, R(int)=0.011). Wumuite has a hexagonal tungsten bronze (HTB)-type structure. The layers of corner-sharing [(W,Al)O]6 octahedra, with the layers oriented normal to the short (3.7713 Å) c repeat and along with the W–O–W links, connect to form a hexagonal ring channel (tunnel). K is distributed in the hexagonal channel. An associated new mineral, tewite, which was discovered in the same area, also has a new tungsten bronze (TB)-type-related structure and has a genetic connection with wumuite in both back-scattered electron (BSE) images and synthetic experiments. The formation of wumuite is likely related to the nearby quartz-vein-type Au mineralization. The mineral was formed by a metasomatic reaction between W-rich hydrothermal fluids and the potassium feldspar in the monzonite.

Late Paleozoic tectonic transition from subduction to post-collisional extension in Eastern Tianshan, Central Asian Orogenic Belt

Abstract Late Paleozoic large-scale transcurrent tectonics and synkinematic intrusions are prominent features in the Eastern Tianshan segment of the southwestern Central Asian Orogenic Belt. However, the spatial and temporal relationship between synkinematic intrusions and crustal-scale shear zones remains unclear. Here we report petrology, geochemistry, and geochronology of the Qiziltag pluton associated with the Kanggur-Huangshan Shear Zone (KHSZ) with a view to characterize the spatial and temporal relationship between synkinematic intrusions and large-scale transcurrent shearing. Field relations and zircon U-Pb ages indicate that the Qiziltag pluton was formed through two stages of magmatism, with earlier stage granitoids (gneissic biotite granite: 288.9 ± 1.9 Ma, biotite monzogranite: 291.5 ± 1.7 Ma, K-feldspar granite: 287.9 ± 3.1 Ma), and later stage bimodal intrusions (biotite quartz monzonite: 278.5 ± 1.8 Ma, gabbro: 278.1 ± 2.3 Ma). The earlier stage granitoids are high-K calc-alkaline, enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs; e.g., Rb, Th, and U), and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti). Combined with their depleted isotopic compositions (εNd(t) = +6.29 to +7.48) and juvenile model ages (TDM2 = 450–610 Ma), we infer that the granitoids were derived from juvenile lower crust in a post-collisional tectonic transition (from compression to extension). The structural and temporal features indicate that the earlier stage (ca. 290 Ma) granitoids formed prior to the regional large-scale dextral strike slip. The later stage bimodal intrusions are dominated by biotite quartz monzonite as the felsic member and gabbro as the mafic component. The biotite quartz monzonite is high-K calc-alkaline with enriched LREEs and LILEs (e.g., Rb, Th, and U), and depleted HFSEs (e.g., Nb, Ta, and Ti), whereas the gabbro is subalkalic with depleted LREEs and HFSEs (e.g., Nb and Ta), resembling normal mid-ocean ridge basalt features. The bimodal intrusions show similar isotopic compositions (εNd(t) = +6.41 to +6.72 and εHf(t) = +9.55 to + 13.85 for biotite quartz monzonite; εNd(t) = +9.13 to +9.69 and εHf(t) = +4.80 to +14.07 for gabbro). These features suggest that the later stage (ca. 280 Ma) bimodal intrusions were derived from partial melting of depleted mantle and anatectic melting of lower crust materials induced by synchronous underplating of basaltic magma in a post-collisional extension. The structural features of the bimodal intrusions indicate that the later stage (ca. 280 Ma) magmatism was coeval with the development of the KHSZ. In conjunction with spatial and temporal evolution of magmatism and sedimentary records of Eastern Tianshan, we infer that transition between the northward closure of the North Tianshan Ocean and subsequent collision between the Central Tianshan Massif and the Qoltag Arc belt occurred at ca. 300 Ma.

The Anatomy of an Alkalic Porphyry Cu-Au System: Geology and Alteration at Northparkes Mines, New South Wales, Australia

The Late Ordovician-early Silurian (∼455–435 Ma) Northparkes system is a group of silica-saturated, alkalic porphyry deposits and prospects that developed within the Macquarie island arc. The system is host to a spectacular and diverse range of rocks and alteration-mineralization textures that facilitate a detailed understanding of its evolution, in particular the nature and controls of porphyry-related propylitic alteration. The first intrusive phase at Northparkes is a pre- to early-mineralization pluton that underlies all the deposits and varies in composition from a biotite quartz monzonite to alkali feldspar granite. Prior to total crystallization, this pluton was intruded by a more primitive quartz monzonite that marks the onset of a fertile fractionation series. Toward its upper levels, the quartz monzonite is porphyritic and locally rich in Cu sulfides. Subsequently, a complex series of synmineralization quartz monzonite porphyries was emplaced. The quartz monzonite porphyry intrusions have a distinct pipe-like morphology and are ubiquitously K-feldspar–altered with a crystal-crowded porphyritic texture. The textures of the quartz monzonite porphyries and common occurrence of porphyry-cemented contact breccias indicate they were forcibly emplaced and of relatively low viscosity. The quartz monzonite porphyries are therefore interpreted as crystal-bearing, silicate melt-aqueous fluid slurries that represent the conduits through which deep-seated magmatic-derived ore fluid was discharged into the shallow crust (1–2 km depth). Each deposit is centered on a multiphase cluster of quartz monzonite porphyry intrusions that drove discrete hydrothermal systems. Initial fluid evolution was similar in all the deposits, with three major alteration facies developed as largely concentric zones around the quartz monzonite porphyry complexes. The innermost zone is host to Cu sulfide ore and dominated by K-feldspar alteration. This transitions outward through a shell of magnetite ± biotite alteration, with pyrite and minor chalcopyrite, to an outer halo of propylitic alteration. Generally, epidote, chlorite, and pyrite are abundant in the most deposit-proximal propylitic zone, with a decrease in the abundance of pyrite, and then epidote, with increasing distance away from deposit centers. Propylitic alteration, particularly within relatively low permeability rocks, is fracture-controlled and a hierarchy of veins is observed. Veins of chlorite-quartz-pyrite ± calcite ± hematite ± epidote ± chalcopyrite (P1) appear to represent the principal fluid conduits. They are surrounded by pervasive and intense alteration halos with a distinct mineralogical zonation from vein-proximal chlorite-sericite (phengite) ± epidote ± pyrite, through hematite-sericite-chlorite ± epidote, ultimately to a vein-distal hematite-albite ± chlorite ± epidote assemblage. These P1 veins are surrounded by regions in which smaller epidote-chlorite ± calcite ± quartz ± pyrite veins (P2) are abundant, again with zoned alteration envelopes: vein-proximal chlorite-sericite (phengite) ± epidote ± pyrite grades out into an epidote-rich zone, which in turn transitions into vein-distal albite-hematite ± chlorite ± epidote. Areas of weakest propylitic alteration, distant from both P1 and P2 veins, are characterized by small epidote-only veinlets (P3) with albite-hematite halos. Mineralogical transitions across the propylitic zone are therefore repeated in the evolution from P1 to P3 veins, as well as in the halos around these veins. It is the overall vein abundance and overlap of associated alteration halos that controls the intensity and appearance of propylitic alteration in most rocks. Such scale invariance and spatial relationships strongly suggest the transition from P1 to P3 veins reflects a broadly decreasing outward flux of (magmatic-derived?) fluid that passed through the fracture network and progressively reacted with country rocks. Further support for this hypothesis comes from crosscutting relationships and Rb-Sr dating of epidote (returning an age of 450 ± 11 Ma), which demonstrate the bulk of propylitic alteration was coeval with mineralization and potassic alteration. Late-stage fluid evolution at each deposit was unique. Much of the E48 orebody, and locally the GRP314 deposit, was overprinted by texturally destructive, white sericite-albite-quartz-alunite ± chlorite alteration. In the E26 deposit and in regions of the GRP314 deposit a series of quartz-anhydrite ± pyrite ± Cu sulfide veins with distinctive, vein-proximal, sericite-dominant alteration halos cuts the primary, deposit-concentric alteration facies. The vein-distal mineralogy of these alteration halos is controlled by their distance from deposit centers, changing from K-feldspar ± biotite in deposit-proximal veins to chlorite ± epidote-albite in depositdistal veins. Late-mineralization quartz monzonite porphyries at E26 and GRP314 also appear to be related to the generation of anhydrite-quartz ± sphalerite veins and a set of quartz-calcite-pyrite-sphalerite ± chalcopyrite ± galena veins. Postmineralization magmatic activity produced relatively primitive and barren monzonite porphyries and younger alkali basalt dikes.

U-Pb geochronology and petrogenesis of intrusive rocks: Constraints on the mode of genesis and timing of Cu mineralization in SWSK area, Lut Block

The South-West Sorkh-Kuh (SWSK) area, part of the Tertiary volcanic-plutonic rocks in the Lut Block, eastern Iran, comprises volcanic rocks intruded by intrusive rocks of hornblende diorite and biotite quartz monzonite compositions. Biotite quartz monzonite has been intensively affected by quartz-sericite-pyrite, argillic and silicification alterations and hornblende diorite shows weak propylitic and carbonate alterations. Disseminated, stockwork and vein-type styles of mineralization are associated with intrusive rocks. The vein-type mineralization with NW-SE trend is hosted by hornblende diorite. The ore in vein-type mineralization consist of quartz, chalcopyrite, pyrite, and secondary Cu-Fe minerals. Disseminated and stockwork mineralizations consist of quartz, pyrite, chalcopyrite, and secondary minerals replacing biotite quartz monzonite and hornblende diorite intrusive rocks. The highest grades in terms of Cu (3%), As (1.3%), Mo (150ppm), Pb (362ppm), Zn (743ppm), and Au (278ppb) are associated with vein-type mineralization. Intrusive rocks causing mineralization, are of metaluminous to peraluminous types with calc-alkaline to high-K calc-alkaline affinity formed in a volcanic arc setting. The enrichment of LILE (Ba, K, Cs, Sr), depletion of HFSE (Nb, P, Ti) and enrichment of LREE relative to HREE indicate the formation of source magmas above a subduction zone. The results of zircon U–Pb dating give constraints on the timing of emplacement of biotite quartz monzonite and hornblende diorite bodies at 40.16±0.72 and 20.1±2.1Ma, respectively. The younger date extends the time span of mineralizations in the Lut Block from Middle Eocene to Early Oligocene (42–33Ma.) reported by previous researchers to more recent Early Miocene times. The initial 87Sr/86Sr ratios and εNd values of both biotite quartz monzonite and hornblende diorite reside in the restricted ranges of 0.7055–0.7066 and −0.33 to +1.88, respectively, consistent with an origin above a subduction zone followed by crustal contaminations of the initial magma compositions, especially with regard to biotite quartz monzonite. The zircon U-Pb ages as well as geochemical and radiogenic isotope data for the intrusive bodies associated with mineralization in this study are in accordance with previous studies on intrusive bodies related to porphyry copper deposits in the Lut Block. In contrast to previous studies, this study further extends the span of mineralization events to at least Early Miocene times.

Two types of ultrapotassic plutonic rocks in the Bohemian Massif — Coeval intrusions at different crustal levels

We present U–Pb zircon age determinations of two Variscan ultrapotassic plutonic rocks from the Moldanubian Zone (Bohemian Massif). Equant, multifaceted zircons without inherited cores from a two-pyroxene–biotite quartz monzonite of the Jihlava Pluton yielded a precise age of 335.12 ± 0.57 Ma, interpreted as dating magma crystallization. The majority of both tabular and prismatic grains from the amphibole–biotite melagranite (“durbachite”) from the Třebíč Pluton plot along a discordia intersecting the concordia at 334.8 ± 3.2 Ma; prismatic zircon grains commonly contain inherited cores and yield an upper intercept age of 2.2 Ga, indicating early Proterozoic inheritance. We therefore suggest that both types of the ultrapotassic plutonic rocks from the Bohemian Massif crystallized at ca 335 Ma, and the previously published ages higher than ca 340 Ma for “durbachites” were biased by a small amount of unresolved inheritance. The ultrapotassic magma emplacement in the middle crust was related to rapid exhumation of a deep crustal segment, considered as isothermal decompression between high-pressure (∼ 340 Ma) and medium-pressure (∼ 333 Ma) stages recorded in granulites. Mineral assemblages as well as external and internal zircon morphology suggest that the Jihlava intrusion was deep and dry, whereas the Třebíč intrusion was shallow and wet. Low ε Hf values of zircons (− 4.4 to − 7.5) in both rock types suggest a similar source with a predominant crustal component. However, inherited grains in the Třebíč melagranite indicate its contamination with crustal material during emplacement, and thus possibly a slower rate of exhumation and/or of magma ascent through the crust.

Endeavour Copper-Gold Porphyry Deposits, Northparkes, New South Wales: Intrusive History and Fluid Evolution

Four economic porphyry copper-gold deposits, Endeavour 22, 26, 27, and 48, occur within the Late Ordovician Goonumbla Volcanic Complex of central-west New South Wales, Australia. Together these deposits have a combined ore reserve of 63.6 million metric tons (Mt) at 1.1 percent copper and 0.5 g/t gold. Mineralization is centered in narrow, pipelike quartz monzonite porphyry (QMP) intrusive complexes. We have recognized at least nine intrusive phases within the Endeavour deposits. These are, in order of emplacement, as follows: a premineral equigranular monzodiorite; early-mineral, equigranular to weakly porphyritic biotite quartz monzonite (BQM) stocks and dikes; synmineral K-feldspar QMP (K-QMP) pipes and dikes; late-mineral augite-biotite–K-feldspar QMP (KA-QMP) intrusions and biotite QMP (B-QMP) dikes; and postmineral basaltic trachyandesite dikes, augite monzonite porphyry dikes, and basaltic dikes. Early-stage biotite-magnetite and propylitic alteration of the host volcanic rocks and K-feldspar alteration of the BQM occurred at each deposit during the intrusion of the BQM stocks and early-mineral B-QMP dikes. Transitional-stage unidirectional solidification textures and other related anisotropic textures formed mostly during the emplacement of K-QMP and KA-QMP intrusions. Main-stage sulfide mineralization at all four deposits is spatially and temporally associated with the K-QMP and, to a lesser extent, KA-QMP intrusions and their associated K-feldspar and sericite-hematite alteration assemblages, and is characterized by multiple generations of stockwork and sheeted quartz, K-feldspar, bornite, chalcopyrite, and gold-bearing veins. Late-stage sericite-quartz-copper-sulfide-carbonate-hematite alteration and vein assemblages formed prior to the emplacement of the postmineral intrusions. Weak to moderate postmineral propylitic alteration assemblages were the last alteration event related to the QMP intrusive complexes. Intrusive activity and associated biotite alteration occurred between 446 and 437 Ma, based on 40Ar/39Ar analyses of biotite and hornblende. Based on microthermometric analyses of fluid inclusions from early-, transitional-, main-, and late-stage veins, the Endeavour deposits formed at depths between 1,000 and 1,700 m below the paleosurface, with a lithostatic pressure regime prevailing throughout the early, transitional, and main stages and near-hydrostatic pressures during the late-stage sericitic events. The first three stages were dominated by high-temperature (460°–>550°C) magmatic-hydrothermal brines with salinities of ~55 to 60 wt percent NaCl ± KCl equiv. Although the late-stage fluids were cooler (350°–400°C) and less saline (~40 wt % NaCl equiv), their compositions are consistent with the magmatic-hydrothermal origin indicated by previous oxygen and hydrogen isotope studies, confirming that the Endeavour porphyry deposits are representatives of the orthomagmatic end member of the porphyry continuum.

Shoshonite‐hosted endeavour 48 porphyry copper‐gold deposit, Northparkes, central New South Wales

The Endeavour 48 copper‐gold orebody is the most recent discovery in the Northparkes Project, located in the Goonumbla district of central New South Wales. The discovery resulted from a small team conducting a systematic appraisal of high‐quality geological and geophysical datasets collected over 20 years of exploration in the Goonumbla district. A key element of the discovery was a rigorous assessment of low‐level aeromagnetic data coupled with persistence in drilling narrow, pipe‐like targets. The Endeavour 48 deposit is a shoshonite‐hosted porphyry copper‐gold deposit. Mineralisation is associated with quartz stockworks and a late‐stage magmatic derived sericitic overprint, and is closely related to at least two stages of quartz monzonite porphyry intrusion. These are genetically related to the biotite quartz monzonite stock (Endeavour 31) which forms the base of the deposit. The mineralisation is hosted by the intrusions, volcaniclastic sandstones, coarse volcaniclastic breccias and latitic (shoshonitic) lavas. The intrusive and volcanic suite are shoshonitic, based on their petrographic and geochemical characteristics. Pervasive biotite‐magnetite ± K‐feldspar alteration forms the earliest alteration phase which is overprinted by sericite, carbonate, albite, hematite, biotite, magnetite ± K‐feldspar related to the main quartz‐bornite stockwork veining. The final alteration assemblage is a strong pervasive sericite‐carbonate‐albite‐clay mineral overprint, which is associated with high‐grade disseminated copper and gold mineralisation. Endeavour 48 lies on Endeavour Linear and the mineralising porphyries, together with quartz veining, follow this trend. A low‐angle fault (Altona Fault) truncates the top of the deposit 50 m below surface.

Petrogenesis of the Ear Mountain tin granite, Seward Peninsula, Alaska

Ear Mountain is part of a belt of Cretaceous tin on the Seward Peninsula of western Alaska. Porphyritic and seriate biotite quartz monzonite and quartz syenite represent early pluton units. These were followed by equigranular and fine-grained two-mica tourmaline-bearing granite. Late units are enriched in B, F, and Na/K and are depleted in Ti, A greisen alteration assemblage of tourmaline-quartz-cassiterite-white mica + or - magnetite + or - pyrite + or - pyrrhotite + or - rutile + or - arsenopyrite is found in the granite and skarns surrounding the pluton.Granite compositions approximate the minimum in the synthetic granite system at a pressure of 0.5 kb. Compositions of fine-grained granite are shifted toward the F-enriched minimum in the synthetic system. Temperature estimates based on feldspar geothermometry range from 624 degrees to 501 degrees C and are close to the solidus temperature in the F- and B-enriched systems. Coexisting Fe-Ti oxide mineral data give temperature estimates of 625 degrees to 675 degrees C and indicate cooling along a buffered path intermediate between the quartz-fayalite-magnetite and nickel-nickel-oxide curves. Biotite compositions (Fe/Fe + Mg = 0.69-0.99) combined with the feldspar geothermometry also indicate that crystallization of the pluton followed a buffer near the nickel-nickel-oxide curve.The tin contents of Ear Mountain granites largely correlate with degree of greisen alteration. Unaltered granites contain less than 80 ppm Sn, whereas thoroughly altered granites contain over 950 ppm Sn. Tin concentrations in unaltered granitic rocks increase progressively from porphyritic to seriate to some equigranular rocks, indicating a progressive concentration of Sn with fractional crystallization. Most unaltered fine-grained granites contain low Sn values, interpreted as results from Sn lost to the evolved vapor phase.The moderately reducing, weakly peraluminous and low calcium conditions during crystallization of the Ear Mountain pluton allowed for concentration of Sn into the melt. High B and F contents in the melt promoted prolonged fractionation of the system and increased concentration of Sn. Evolution of a vapor phase removed Sn from the granitic melt and reaction of this vapor with the granite and the skarn produced the Sn-enriched greisen alteration. There is no evidence that Sn was remobilized from Sn-enriched biotites into greisen.Greisen development is not extensive at present levels of erosion. Abundant cassiterite in placers surrounding the pluton suggest much of the greisen zone has been removed by erosion. The apical parts of other tin granites on the Seward Peninsula, such as Lost River and Kougarok, have well-developed greisen zones. Ear Mountain probably resembled these systems prior to erosion.

Petrologic Significance of Zircon Variation in the Tunk Lake Granite, Southeastern Maine

Zircon varies systematically in the gradational rock units of the concentrically zoned Tunk Lake pluton. Average zirconium content decreases from 660 ppm in marginal, hypersolvus, aegirine augite-hornblende granites to 170 ppm in the biotite quartz monzonite core. Zircon decreases in mean length from 0.24 mm in the margins to 0.13 mm in the core. Core zircons tend to be partially metamict and less elongate and have more complex crystal form than zircons of the margins. Frequency distributions of zircon characteristics show that marginal rocks contain zircon populations similar to the average population calculated for the pluton but with additional large crystals, and that core rocks have populations similar to the average but with large crystals removed. Broken and clustered zircons support gravity settling and magma tic convection as causes of zircon variation. Zircons accumulated in the margins of the pluton where downward-moving currents of magma were deflected inward. Zircon distribution provides info...

Compositional Variation in the Tunk Lake Granite Pluton, Southeastern Maine

The Tunk Lake granite pluton is exposed over a circular area of 70 square miles in southeastern Maine. It intrudes the Ordovician (?) Ellsworth Schist, the middle Paleozoic Bays-of-Maine gabbroic-granitic complex, and Middle Devonian biotite granite and quartz monzonite. From the margin of the pluton to the core, six concentric gradational zones are recognized: 1 magnetite-aegirine augite granite chill zone; II hornblende-aegirine augite granite; III hornblende granite; IV hornblende-biotite granite; V biotite granite; VI biotite quartz monzonite. Modal and chemical data for the pluton show regular compositional variation from margin to core. Quartz and oligoclase increase inward, whereas total feldspar, perthite, percentage of albite in perthite, and total mafics and accessories decrease. The dominant mafic mineral varies inward according to the series magnetite-aegirine augite-hornblende-biotite. SiO2 increases toward the core of the pluton paralleling the quartz variation, whereas Al2O3, Na2O + K2O, FeO + Fe2O3, and CaO decrease corresponding to the variation in total feldspar and total mafics and accessories. Emplacement of the pluton at an assumed depth from one to three miles occurred by piecemeal stoping along marginal zones of ring fracturing and by cauldron subsidence or stoping of large blocks in the central zones. The magma at the time of intrusion is assumed to have had a composition, indicated by a calculated average for the pluton, approximately equal to the biotite granite of zone V, a volatile content from 2 to 3 percent, and a temperature of 800° C. Crystallization began in the outer portions of the pluton with precipitation of magnetite and aegirine augite followed by sodium-rich alkali feldspar and quartz. Exsolution of feldspar was followed by the partial replacement of primary mafic minerals by hornblende and biotite. During crystallization, silica-rich aqueous fluids migrated from the margins to the center of the pluton. The marginal rocks were left quartz-poor, and feldspar- and mafic-rich. The core rocks became quartz-rich. In the central zones additional water lowered crystallization temperatures allowing oligoclase, hornblende, and biotite to form as primary minerals and also facilitated late-stage recrystallization. Recrystallization fluxed by hydrous fluids accounts for fine-grained intergranular material in the central granites and also for sheets, lenses, and irregular bodies of fine-grained rock throughout the pluton. The pluton is related to several circum-Atlantic groups of intrusions which are similar in size, shape, petrography, and emplacement. Mineralogical and compositional variations in the granitic rocks of the White Mountain plutonic-volcanic series of New Hampshire and the “Younger Granites” of Northern Nigeria are similar to trends observed in the zones of the Tunk Lake pluton. In the Tunk Lake body, the variations occur within a single, gradational intrusion with a relatively simple crystallization history which may closely parallel those of the White Mountain and Nigerian intrusive complexes.

Virginia Dale Precambrian Ring-Dike Complex, Colorado-Wyoming

The Virginia Dale ring-dike complex is a roughly circular granitic feature bout nine miles in diameter in the northern Front Range, straddling the Colorado-Wyoming boundary. The deeply eroded complex is a part of a large batholith of Sherman Granite, of Precambrian age, and discordantly intrudes metamorphic rocks. It consists of four main units: an outer granite ring dike, which merges to the north with the main Sherman batholith; a composite zone of metamorphic rocks and mafic igneous rocks; and two inner zones of quartz monzonite, comprising the core. The outer ring dike, about a half mile wide, consists of coarse equigranular hornblende-biotite granite, and the two core zones consist of porphyritic biotite quartz monzonite. Both the granite and quartz monzonite are facies of Sherman Granite. Between the outer ring and the core in the southern portion of the complex is a composite zone consisting of pre-Sherman metamorphic rocks, and of diorite, andesite, quartz monzonite, and hybrid rocks. These rocks show much evidence of brecciation, mechanical mixing accompanied by flow, and metasomatism. Field relations and petrographic analysis suggest that the diorites are of hybrid origin. Chemical analyses, petrographic data, and trend analysis suggest that granite in the outer ring dike of the complex is of composition slightly different from that of granite outside the complex, and that it crystallized at higher water pressure. The quartz monzonite may have crystallized at even higher water pressure than that of granite and in addition was contaminated by mafic material. Relatively high magma volatile contents, as well as hybridization, seem to be characteristic of central complexes, even where erosion has been as deep as in the Virginia Dale complex. Gravity data indicate that the core did not form by cauldron subsidence. Rather, it appears to be a dome emplaced along old ring fractures. The outer ring dike may be a collapse structure, although the batholith to which it is connected to the north may have been emplaced as large sheets.

The diversity of Tanohata granitic mass, northern Kitakami mountains, Iwate Prefecture

The Tanohata granitic mass, northern Kitakami mountains, Iwate Prefecture, shows various rock facies. The granitic mass is classified into six types by the rock facies as follows. 1) Moichi type - hornblende biotite granodiorite 2) Hagiu type - hornblende biotite granodiorite 3) Otomo type - biotite granodiorite 4) Kawaguchi type - biotite hornblende granodiorite 5) Otanabe type - hornblende biotite quartz monzonite 6) Shimoakuka type - hornblende biotite quartz monzonite It is concluded that the Tanohata granitic mass is the composite one, being the product of successive intrusions of the above-mentioned types and the order of intrusions is as follows, that is, at first the Kawaguchi body occurred during the last period of orogeny, the second is the Moichi and then the Hagui and the Otomo intruded successively. The Otanabe erupted in the north contemporaneously with the Otomo, and at last the Shimoakuka occurred as a separated minor body intruding into the Otanabe with very sharp boundary between both of them.