Baddeleyite u-pb zircon dating, uranium–lead dating
Widely distributed ilmenite grains occur closely associated with hornblende Figs. Zircon overgrowths are generally small, sporadic, and occur mostly in the fractures and margins of baddeleyite Fig. These fission tracks inevitably act as conduits deep within the crystal, thereby providing a method of transport to facilitate the leaching of lead isotopes from the zircon crystal.
Albitization of oligoclase involves a fluid phase that introduces Na and Si and releases Ca and Al Engvik et al. Subsequent medium- to high-temperature hydrothermal alteration produced secondary zircon as intergrowths and overgrowths on baddeleyite.
According to the classification of Leake et al. The standard M was conventionally analyzed prior to the unknown samples on the instrumental test target.
In areas with a high concentration of the parent isotope, damage to the crystal lattice is quite extensive, and will often interconnect to form a network of radiation damaged areas. Plagioclase is euhedral and tabular, and has undergone epidotization. Primary zircon formed later during rapid cooling of the late-stage, more fractionated melt enriched in silica during its final intrusion into volcanic rocks of the Majiahe Formation. Low silica activity during early-stage crystallization is also supported by the lack of inherited zircon. These secondary zircon rims on baddeleyite are considered to be of hydrothermal origin.
The isolated zircon grains are best explained as a late-stage and rapidly crystallized phase, suggesting that the early-stage magma was zircon undersaturated. Zirconolite is distributed in plagioclase and quartz and was replaced by titanite along their margins Figs. The monzonite porphyry of this study intruded the Majiahe Formation and is locally covered by Cenozoic sediments in Duoyang area, the southeastern part of Xiaoshan area Fig. Apatite usually occurs at the margin of altered hornblende Figs.
Common lead corrections were made using the measured Pb. The granophyric texture formed by undercooling during emplacement, and the hydrothermal alteration could have taken place nearly at the same time Han et al. These groundmass minerals appear to nucleate on preexisting feldspar phenocrysts Figs.
Previous age data for the volcanic rocks based on K-Ar, Rb-Sr analyses of minerals or whole-rock samples, conventional multigrain U-Pb zircon analyses, and several Sm-Nd analyses Qiao et al. Where crystals such as zircon with uranium and thorium inclusions do not occur, a better, more inclusive, model of the data must be applied. Considering that zircon solubility decreases with decreasing temperature Wilke et al. Hornblende was intensively replaced by chlorite and biotite, and ilmenite associated with hornblende was generally replaced by titanite along its margins Figs.
Crystallization of Baddeleyite Baddeleyite tends to crystallize under silica-undersaturated conditions, and thus is rarely seen in quartz-bearing intermediate to acidic igneous rocks. They commonly show weak parallel striations and occur as polysynthetically twinned crystals. Unlike the late-stage magmatic zircon, the secondary zircon rims in the studied porphyry Figs. Baddeleyite grains in the studied monzonite porphyry occur as inclusions in most other minerals, indicating that they formed at the early stage of crystallization.
Hydrothermal Alteration The monzonite porphyry of this study was intensively altered by hydrothermal fluids. Alteration of hornblende can release the necessary Ca for apatite and titanite growth during chloritization. In addition, highly evolved magmas often possess large amounts of dissolved volatiles e. The monzonite porphyry samples are granophyric Figs.
Albitization and epidotization commonly occur during medium- to high-temperature hydrothermal alteration Li et al. Hence, our conclusion is that the porphyry was altered soon after emplacement by medium- to high-temperature hydrothermal fluids enriched in Si, Na, F, and Cl.
Needle-shaped zircon grains are observed growing along the interface of intergrown K-feldspar and quartz Fig. Fine-grained apatite crystals tend to be distributed on the margins of hornblende Figs. Quartz distributed in K-feldspar varies in habit from cuneiform to vermicular and has a parallel to plumose and radiating texture Figs. Apart from baddeleyite and zircon, Zr also occurs in hornblende and ilmenite in the studied porphyry.
Only a few baddeleyite grains included in altered hornblende have continuous zircon rims Figs. Euhedral, platy baddeleyite crystallized early from the primitive, least-fractionated porphyry magma with lower silica activity.
Only a few baddeleyite grains included in altered hornblende have complete zircon coronas. After fractionation of early-stage Zr-rich minerals, including baddeleyite, ilmenite, and hornblende, definiciones chistosas yahoo dating Zr concentrations in the residual melt may have been far below the saturation value. Many baddeleyite grains are corroded by secondary zircon overgrowths internally Figs.
In the granophyric intergrowths, K-feldspar stringers that emanate from the same seed porphyry show optical continuity with each other and have the same extinction direction. Survival of baddeleyite as the magma became silica saturated implies a rapid cooling process. The monzonite porphyry intruded volcanic rocks of the Majiahe Formation nearly simultaneously with their eruption.
Some quartz grains are round or triangular Figs. These observations indicate that zircon did not crystallize until final emplacement. Some zircon crystals are striated and also show polysynthetic twinning. Similar alteration and resetting of hornblende and plagioclase thermobarometry was described in Anderson et al.
As the porphyry samples were intensively altered by hydrothermal fluids, amphibole and plagioclase pairs with smooth surfaces were chosen for temperature and pressure estimation. The granophyric intergrown K-feldspar and quartz form the groundmass of the monzonite porphyry. Hornblende is usually replaced by chlorite and biotite from the core to rim. In this study, zircon and baddeleyite were successfully extracted from three mineralized samples of the J-M Reef.
Hornblende and ilmenite are also the major carriers of Zr Rubin et al. Subsequent medium- to high-temperature hydrothermal alteration affected most minerals at subsolidus conditions. In general, hornblende contains highly mobile Zr Rubin et al.
The introduced epidote grains distributed in late-stage quartz veins Figs. The alignment of the discordant data indicates that these zircon grains belong to the same age group as the concordant spots. Magmas that ascend close to the surface without eruption can be undercooled by rapid rising of magma and heat conduction with cold country rocks. Intergrown K-feldspar and quartz in the granophyric groundmass often have the same extinction direction, indicating concurrent growth. Some needle-shaped zircon grains are distributed along with elongate K-feldspar or on the interface of quartz and K-feldspar in the granophyric groundmass.
Undercooling of magma may be related to saturation and exsolution of these volatiles with depressurization during magma emplacement Lowenstern et al. Holland and Blundy recalibrated the thermometer by extending the data set and taking nonideal mixing both in hornblende and plagioclase into account.
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