Mineral compositions of magmatic dikes cutting across the Khe Phen granites (Huong Tra, Thua Thien Hue, Central Vietnam)

The Khe Phen granite quarry located in Huong Tra district (Thua Thien Hue province) has been confirmed as a part of the Ba Na granitoid complex (G/K2bn), mostly composed of two-mica granite and porphyritic granite. Field survey data show that the granites here are cut across by five distinct narrow dikes (about 50-70 cm wide) including granite pegmatite, granite aplite, aplite, granodiorite and lamprophyre diorite. Mineral compositions of the granite pegmatite and aplite dikes are similar with those of the host granite, which are mainly comprised of quartz (27-35 %), orthoclase (45-58 %), plagioclase (4-15 %), biotite (1-3 %) and a few opaque minerals. Meanwhile, the granodiorite and lamprophyre diorite dikes are melanocratic and compositionally much more mafic, particularly lamprophyre diorite, evidenced by a presence of hornblende (50-55 %), plagioclase (33-40 %), quartz (3-15 %), calcite (5-17 %)... Origin and emplacement age of the latter dikes have not been reported so far, and thus are needed for further studies based on geochemical and isotopic data.


Introduction
Magmatic dikes form when magma intrudes into and crystallizes in cracks as a sheet intrusion, either cutting across layers of rock or through a contiguous mass of rock [1]. Dikes are tabular, often vertical or steeply dipping sheets that cut across the trend of structure (for example, bedding) of invaded rocks [2]. A dike set is composed of several parallel dikes.
They have a wide range of rock compositions and commonly have a porphyritic texture. The width of dikes varies from a few centimeters to more than ten meters. Studying mineralogy, isotopic compositions, and geochemical systematics of dikes could reveal petrogenesis of host rocks as well as tectonic setting, emplacement evolution and geodynamic significance [3,4,5].
The Khe Phen granite quarry belongs to the Ba Na complex [6], composed of one intrusive phase and one dike phase. The former includes biotite granite, muscovite-bearing biotite granite, two-mica granite, granodiorite, and tonalite. The latter includes aplite and tourmaline-bearing pegmatite [6,7,8,9]. The current field survey has revealed that besides leucocratic dikes as described above, the granites are also cut across by melanocratic dikes, which have not been mentioned so far. Therefore, the purpose of this paper is to provide initial observations on petrographic and mineral description, which is necessary for further studies on the origin and the emplacement of such dikes.

Geological features
The area is located about 4 km to the southwest   Biotite is sparsely present in the rock as irregular tabular or flaked crystals with a dark-brown color and a perfect cleavage plane. Many of the biotite have been partly altered to chlorite (Fig. 4a, b).  rare, occupying up to 2% of the rock (Fig. 2b, c, d).
Quartz is anhedral inequigranular, light-gray, colorless-transparent. All of the quartz has no cleavage, a low relief, and a clear surface.
Undulose extinction is commonly observed in the rotation of the stage. In general, the quartz crystals are regularly distributed in the rock; at places, they are concentrated as minute clusters, resulting in graphic texture (Fig. 4d). boundaries or developing on feldspars' surface ( Fig. 4c, d).
Quartz is anhedral, light-grey colored, inequigranular grains with colorless-transparence, very low relief, and glossy surface. They largely distribute in the sample; some are developed along feldspar grains' boundaries (Fig. 4e).
Orthoclase occurs in the form of grey subanhedral tabular-shaped crystals. Their cleavage is not discernible, whilst twinning is relatively good. The orthoclase crystals have partly been altered after a pelitic process and surrounded by aggregates of fine-grained quartz (Fig. 4e).
Plagioclase is grey sub-anhedral stubby prismatic grains, which are characterized by a perfect lamellar twinning. The crystals have intergrown with the orthoclase and have been modified by a weak saussuritization process (Fig. 4e). Biotite is present in the samples as fine flakes with perfect cleavage. They largely distribute along the rest minerals' grain boundaries, particularly along quartz and feldspar crystals, and have been altered by a weak chloritized process resulting in a typical greenish-brown color (Fig. 4e).
Granodiorite is a massive porphyritic rock, characterized by a greenish-black color due to a strong alteration to chlorite and sericite of primary minerals such as plagioclase (Fig. 2d). Lamprophyre has massive poikilitic texture and is greenish-black in hand specimen (Fig. 2d).
The rock is also called spessartite, belonging to an   [11,12] and Le Duc Phuc [13], as well as geological setting of the area during Mesozoic stage.