花岗岩中的钾长石巨晶:以南岭佛冈花岗质杂岩体中微斜长石巨晶为例

The K-feldspar Megacrysts in Granites: A Case study of Microcline Megacrysts in Fogang Granitic Complex, South China

野外地质观察和岩石显微结构研究表明 ,佛冈花岗质杂岩体中微斜长石巨晶是岩浆结晶的产物 ,不是交代斑晶 ,也不是变斑晶。它们与基质中的微斜长石构成双峰式粒径 ,反映了岩浆的两阶段结晶历史。由于微斜长石是钾长石的低温变体 ,因此一种可能的机制是佛冈花岗质杂岩岩浆在侵位和基本固结后 ,冷却缓慢 ,使早结晶的正长石转变为微斜长石。此外 ,在新近的研究中 ,还发现了罕见的、但在佛冈花岗质杂岩体中为数不少的“十字”贯穿式双晶微斜长石 ,以及微斜长石巨晶的“环斑”和“珠边”结构。

K feldspar megacrysts are very common in granitic rocks. They could be either microcline or monoclinic orthoclase, and their origins are still debated. Microcline megacrysts, 2 to 12 cm long, are easily found in Fogang granitic complex, South China. They are usually euhedral in shape, common in simple Carlsbad twinning or grid twinning, but quite often in cross interpenetration twinning. They are orientated with long axes more or less parallel to the flow direction of the magma. Tiny minerals such as plagioclase, biotite, quartz etc. are poikilitically enclosed in them. In addition, the microcline megacrysts sometimes are surrounded by a thin plagioclase film forming mantled feldspar, and sometimes intergrown with quartz forming 'pearl margin'. More petrographic features of subsolidus exsolution and oscillatory zoning can also be easily found under microscope. X ray powder diffraction analysis indicates that the triclinicity of the microcline megacryst is 0.963, t 1o+t 1m=0.970, t 2o=t 2m=0.015, and therefore it is highly Si Al ordered microcline. The microclines in groundmass of Fogang granitic complex are commonly in simple Carlsbad twinning or grid twinning and exsolution texture, but with much smaller size and interstitial irregular shape. All the above mentioned petrolographic features show that the microcline megacrysts in Fogang granitic complex were crystallized from silicate melt. In order to further document the origin of the microcline megacrysts, we have also studied the microgranitoid enclaves in Fogang granitic complex as same microcline megacrysts have been found in these enclaves. These enclaves of quartz diorite or granodiorite composition with round shape, needle like apatite and biotite, ocular quartz and poikilitic texture, are typical magmatic enclaves formed by magma mixing. Therefore, the microcline megacrysts are really xenocrysts from the host magma. The detailed magmatic crystallization history could be addressed by using Swanson's (1977) experimental diagram. At ca. 850℃, plagioclase, biotite and K feldspar crystallized simultaneously, but the K feldspar has a lower nucleation density and higher growth rate than other minerals. Then plagioclase and biotite could be enclosed by the faster growing K feldspar, or simply were overtaken by the latter. However, as the mineral inclusions are usually concentrated at the centers of the K feldspar, the outer parts of the K feldspar were formed at a later stage. At ca. 800℃, quartz started its crystallization, and its nucleation density was close to that of K feldspar, but the growth rate of K feldspar was much faster than that of quartz, and their intergrowth formed the 'pearl margin' of the K feldspar. At ca. 600℃, even shaped and small sized grains were crystallized forming groundmass. The fact that the K feldspar megacrysts and groundmass are microcline rather than orthoclase is significant for understanding the cooling history of the granitic complex. A possible mechanism is that the triclinic microcline was transferred from the monoclinic orthoclase during the slow cooling history.