International Geologiical Congress - Oslo 2008


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MPI-01 General contributions to igneous petrology


Petrogenetic modelling of the vertically zoned Cretaceous pohang epizonal intrusive rocks, SE Korea


Young-Kook Hong, Korea Institute of Geoscience & Mineral Resources (Republic of Korea)


Various plutonic rocks in the Pohang C-borehole are shown to be closely related in space, time and composition. The deep-drilling cores of about 3 km from the C-well provide a remarkable vertical cross-section of the regional geology including 1.7 km of the cogenetic epizonal plutonic rocks.
The mineralogically and petrochemically zoned Pohang intrusive rocks are divided into three plutonic rock types such as diorite, granodiorite and granite, and become progressively more evolved downward. The plutons are generally fine to medium-grained at the margin and grade into medium to coarse-grained toward the center.
Petrogenetic modelling of trace and rare earth elements variation indicates that two distinct magmas are involved in the Pohang plutonic rocks. The vertical zonation of the Pohang plutons can be explained by multiple intrusions with some time interval such as dioritic and granodioritic magmas from the same parental magma rather than a continuous fractionation from a single dioritic magma. As the results of trace element modelling with Rb, Sr and Ba, the granodioritic magma was mainly controlled by crystallization of hornblende, plagioclase and K-feldspar during in situ fractional crystallization to form the more evolved granite. The fractionation of hornblende with some plagioclase was the predominant process in the parental magma during the formation from the dioritic to granodioritic magma.
But, the decreasing total REE contents, relatively constant (Ce/Yb)N ratio and slightly increasing Eu negative anomalies from diorite to granite through granodiorite can not fully explained by simple Rayleigh fractionation in REE modelling. The peculiar REE patterns could result mainly due to fractionation of particular minerals such as hornblende and accessory minerals during magmatic evolution, since hornblende showing high distribution coefficients for all the REEs in the silicic magma.
From the geochemical point of view, all the subvolcanic intrusive rocks display a typical calc-alkaline chemical character showing the imprint of "volcanic-arc" geodynamic environment. The estimated crystallizing pressure during the emplacement and consolidation for the granodiorite by the empirical hornblende geobarometer is no more than 1 kbar equivalent with about 3 km of subsurface depth.
The radiometric K/Ar age determinations on the hornblendite dyke, which could be closely related with cumulate phase of the parental dioritic magma, intruding into the Cretaceous tuffaceous rocks at the depth of 1240 m reveal as 117 ± 2.62 Ma even if the age is the minimum age due to later intrusion of dioritic magma, which implying that the volcanic activity of the Yoocheon Group in the Gyeongsang sedimentary basin may be started from at least the early Cretaceous period, Neocomian. The whole rock sample from diorite at the 1780 m borehole depth shows 86.87 ± 6.94 Ma by K/Ar dating.


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