Age Dating and Geochemistry of Intrusive Igneous Rocks in Bokeo and Luong Namtha Provinces, Lao PDR

Abstract

The igneous rocks from the Bokeo and Luong Namtha provinces, northwestern Lao PDR are important for constraining the tectonic evolution of the Paleo-Tethys ocean. The investigated area belongs to the Sukhothai zone which is also known as Eastern Granitoid Belt in Thailand and is linked to the Lancangjiang zone in southwestern China. The intrusive rocks in the study area can be separated into two main groups based on geochemical composition: (1) gabbro to diorite group and (2) granitoid group (1) The gabbro-diorite group can be further divided into three subgroups: (I) gabbro dike, (II) gabbro pluton, and (III) diorite suites. The mineral assemblage of the gabbro dike suite was plagioclase + clinopyroxene + opaque mineral. It was described by an ophitic texture with fine grained crystals. It was a slightly enriched in LILEs and LREEs and slightly depleted in HFSEs. The geochemical characteristics of the gabbro dike suite represent a magma derived from a depleted continental lithospheric mantle with contamination from subduction components. The gabbro pluton and diorite suite yielded a zircon U–Pb age of 232.2 ± 3.1 Ma which is related to post-collision after the closure of the Paleo-Tethys. The typical mineral assemblage of the gabbro pluton and diorite suites were plagioclase + clinopyroxene + hornblende ± orthopyroxene ± quartz ± biotite + opaque mineral ± apatite. The gabbro pluton and diorite suites were illustrated by a seriate texture with zoned plagioclase and sieve-textured amphibole. Small quantities of clinopyroxene and orthopyroxene were also discovered as tiny subhedral crystals with amphibole rims. In the chondrite normalized diagram, no noticeable negative Eu anomaly is observable suggesting that plagioclase fractionation did not occur. The primitive mantle normalized diagram proved Nb and Ta negative anomalies. They are enriched in LILEs, depleted in HFSEs, have a high Th/Nb ratio. The gabbro pluton and diorite suites represent melts derived from a subduction setting. Furthermore, CaO/Al2O3, La/Ta, and La/Nb ratio indicate lithospheric mantle source. However, the various chondrite normalized pattern and the value of some trace elements indicate heterogeneous melting of the peridotite source. They can be divided into gabbro-diorite I and gabbro-diorite II. The gabbro-diorite I was enriched in LREEs and depleted in HREEs compared to the gabbro-diorite II. The geochemical data and trace element modeling suggest that the melt source of the gabbro-diorite I and gabbro-diorite II are garnet-spinel and spinel peridotite, respectively. The melt composition of gabbro-diorite I and gabbro-diorite II indicated 10% mixing between primitive mantle and slab-derived component (fmelt=0.10) and 20% mixing between depleted mantle and slab-derived component (fmelt=0.10), respectively. The AFC model of the gabbro-diorite I (fcry = 0.60-0.10) and II (fcry = 0.40-0.10) suites indicate assimilation fractional crystallization evidence in rock samples. Trace element and REEs pattern of fmix = 0.60-0.10 mixing modeling suggested magma mixing between gabbro-diorite I and alkaline-rich granitoid. (2) The granitoid group can be classified as (I) alkaline-rich granitoid (Potassium feldspar rich-granitoid suite), (II) alkaline -poor granitoid (Potassium feldspar poor-granitoid suite), and (III) cordierite bearing granitoid suite.The alkaline-rich granitoid sample from Bokeo and Luong Namtha provinces yields a zircon U–Pb age of 231.0 ± 3.1 Ma. The samples contained the mineral assemblage quartz + plagioclase + K-feldspar + biotite ± opaque mineral ± apatite ± zircon ± allanite. They exhibited a seriate texture, perthitic potassium feldspar grains, and zoned plagioclase. Major elements are typical for calc-alkaline granitoids. Most of the alkaline-rich granitoid samples display a well developed negative Eu anomaly which indicates plagioclase crystal fractionation. The rock samples typically illustrated a wide range in Rb/Sr, CaO + MgO + FeOt + TiO2, and Al2O3 +MgO +FeOt+TiO2. These points display partial melting of juvenile basic lower crust mixed with a greywacke source. The alkaline-poor granitoid suite yielded a zircon U–Pb age of 250.8 ± 3.4 Ma, implying that these rocks were generated in a magmatic arc system during the Early Triassic. The alkaline-poor granitoid suite did not display a negative Eu anomaly suggesting no plagioclase crystal fractionation. The samples had a narrow range of Rb/Sr, CaO + MgO + FeOt + TiO2, and Al2O3 +MgO +FeOt+TiO2, high ratio of LaN/YbN, and Mg# indicating melting of a metabasaltic or eclogitic source. Thus, the alkaline-poor granitoid suite is associated with high-pressure melting where garnet remains in the metabasaltic source as refractory phase. The cordierite bearing granitoid suite yielded a zircon U–Pb age of 244.4 ± 3.2 Ma. The cordierite bearing granitoid suite had as a distinctive feature cordierite crystal as a phase. The cordierite bearing granitoid suite displayed a narrow range in CaO/Na2O and a high LaN/YbN ratio. In diagram CaO + MgO + FeOt + TiO2 vs. CaO/(MgO + FeOt + TiO2) and Al2O3 +MgO +FeOt+TiO2 vs. Al2O3 /(MgO +FeOt+TiO2), nearly all of the samples plot into the field of an amphibolitic source. The petrological, geochemical and geochronological results of this study suggest that the alkaline-poor and cordierite bearing granitoid suite rocks from Bokeo and Luong Namtha provinces represent mainly a volcanic arc setting during Early Triassic. The gabbro pluton- diorite and alkaline-rich granitoid suites may be related to a post-collision event following the closure of the Paleo-Tethys during Late Triassic.