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•Position-by-position t-T paths are reconstructed from thermochronological data.•Position-specific t-T paths reflect the formation history of the pluton.•It is a new method for ...understanding of fracture populations in granite.•The t-T path evaluation provides key insight to investigate fracture frequency.
Understanding the locational differences of time-temperature (t-T) paths within the granite contributes to clarifying the detailed formation history, especially the thermal evolution, of the entire pluton after the incipient intrusion of the granitic magma into the shallow crust. The position-by-position t-T paths of fifteen samples of the Toki granitic pluton, central Japan, are reconstructed from thermochronological data, including zircon U–Pb, biotite K–Ar, zircon fission-track (ZFT), and apatite fission-track (AFT) ages, FT length characterization, and FT inverse calculation. In case of sample No. 1, the t-T path consists of zircon U–Pb ages (74.0 ± 1.1 Ma and 72.6 ± 1.7 Ma), a biotite K–Ar age (73.0 ± 1.8 Ma), a ZFT age (57.2 ± 2.3 Ma), an AFT age (41.5 ± 2.7 Ma), and the FT inverse calculation. The t-T paths show position-specific characteristics that reflect the history of the pluton after the incipient intrusion of the granitic magma into the shallow crust. Shallow parts of the western and northeastern margins of the pluton show rapid cooling from the intrusion stage to the ZFT partial annealing zone (PAZ), and thus represent position-specific characteristics in relation to crustal assimilation in the intrusion/emplacement processes. The cooling behavior around the condition of AFT PAZ was interpreted as a reflection of regional tectonic exhumation of the pluton. This study also presents useful knowledge to evaluate fracture populations in a granitic pluton. The relationship between the position-specific t-T path and fracture frequency (fracture number per unit interval) provides a criterion for evaluating the fracture population of the granitic pluton, called ‘thermochronological evaluation’, which is a key insight to investigating fluid flow and mass transfer in the rock mass. The t-T path shows whether cooling was slow or rapid from thermal conditions of solidification/crystallization to the biotite K–Ar closure temperature; parts of the pluton that cooled slowly correspond to parts with high frequencies of fractures, and vice versa. The absolute value for duration between the zircon U–Pb age evaluated from the oscillatory-zoned area (OZ age) and biotite K–Ar age shows high correlation with fracture frequency (R2 = 0.89), and thus gives a quantitative indicator for evaluating fracture frequency; longer duration accompanies higher frequency. A combination of thermochronological evaluation, the local cooling indicator, and the lithofacies effect can be applied as potential diagnostic tools for detailed evaluation of the fracture population.
The three-dimensional spatial variations in the cooling pattern of the Toki granitic body, a zoned pluton in Central Japan, have been evaluated quantitatively by thermochronology using cooling age ...determination based on the different closure temperatures for target mineral species. The Toki granite has hornblende K–Ar ages of about 74.3 ± 3.7 Ma (
N
= 2; closure temperature of 510 ± 25°C), biotite K–Ar ages of 78.5 ± 3.9 to 59.7 ± 1.5 Ma (
N
= 33; 300 ± 50°C), and zircon fission-track ages of 75.6 ± 3.3 to 52.8 ± 2.6 Ma (
N
= 44; 240 ± 50°C). The spatial variation in the biotite K–Ar age is similar to that in the zircon fission-track age in samples collected from 11 boreholes and seven outcrop sites in the Toki granite, indicating that cooling was effectively from the roof and also from the northwest margin. This cooling pattern shows a strong correlation with the Alumina Saturation Index (ASI) distribution of the body. Larger ASI values correspond to earlier and more rapid cooling after emplacement and smaller value to slower cooling. Toki granite was effectively cooled from the peraluminous regions where assimilation of country sedimentary rock was most extensive.
