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Syn-magmatic shearing: Guojialing Granite, Jiaodong Peninsula, Shandong Province, China

 

Roberto Weinberg
School of Geosciences, Monash University, Australia

 

 

 

 

These photos were taken atthe Cang Zhuang quarry of the Guojialing Granite (N37 26 478, E120 11 441), Jiaodong Peninsula, Shandong Province in China. The 130-126 Ma granite is a Bt-Hbl quartz monzonite and granite with K-feldspar megacrysts, commonly 1- to 5 cm long, up to 15 cm in lengt, and accessory titanite. The rocks shows simple shearing at late stages of magmatism leading to the following features: a) leucocratic melt accumulation at strain shadows of K-feldspar megacrysts, b) necking in between megacrysts due to extension, and melt migration to neck zone, c) magmatic banding with K-feldspar megacryst and felsic bands interlayered with biotite-amphibole rich bands, d) preferred orientation of long axis of megacrysts parallel to the S plane of deformation, e) syn-magmatic fracturing of megacrysts with fractures filled by granitic matrix, f) sigma-shaped megacrysts with evidence of solid state deformation. The syn-magmatic shear plane (C-plane) strikes 250/30 NW, a lineation on this planes trends 355 and the sense of shear is top-to-the north normal movement. This foliation, being syn-magmatic, developed at 130-126 Ma, the age of the Guojialing magmatism (see Qiu et al., 2002, Min. Dep., v. 37). The Guojialing plutons typically has low SiO2 content (67 wt%), very high Ba (>2,800 ppm), low Rb/Sr ratio (<0.1) and low ?Fe/(?Fe+Mg) ratio (0.4) of biotite (Qiu et al., 2002).
Megacrysts of K-feldspar Megacrysts of K-feldspar
a) Zoned K-feldspar megacryst wrapped by a faintly foliated matrix. b) Sigma-shaped K-feldspar megacryst with asymmetric tails of felsic granite. Note megacryst in the middle right-hand-side has rotated and is surrounded by a sigma-shaped matrix. Top to the right movement sense.
Megacrysts of K-feldspar Megacrysts of K-feldspar
c) Sigma-shaped K-feldspar megacryst with tails of felsic granite. Three K-feldspar crystals in the central part are linked by a felsic matrix band indicative of extension between crystals during solidification and seggregation of magma into the necks. Asymmetry of individual crystals and of the combined object indicates top-to-right movement sense. d) Sigma-shaped K-feldspar megacryst with tails of felsic granite, asymmetry indicates top-to-right movement sense.
Megacrysts of K-feldspar Megacrysts of K-feldspar
e) Sigma-shaped K-feldspar megacryst with tails of felsic granite. Note mafic-rich band on the side of the megacryst parallel to the main foliations plane. Top-to-right movement sense. f) Two K-feldspar megacrysts with felsic granite tails, the grain on the right rotated slightly in relation to the one on the left. Note broken piece of megacryst on the left=hand-side. Matrix defines two foliations and individual grains are sigma-shaped. Top-to-right movement sense.
Megacrysts of K-feldspar Megacrysts of K-feldspar
g) Tiled and deformed K-feldspar sigma clasts (with length parallel to S-plane), indicative of top-to-right movement sense. Megacryst in the upper right has been broken and fracture was filled with magma indicating syn-magmatic fracturing. Note darker and lighter bands throughout the rock, h) Like (g), central grain has been broken and fracture was filled with magma indicating syn-magmatic fracturing. Note the bands richer in mafic minerals deflecting above and below the central, fractured crystal, and bands of more felsic granite in the shadows of the megacrysts also indicative of syn-magmatic deformation.
Megacrysts of K-feldspar Megacrysts of K-feldspar
i) Rotated and deformed K-feldspar sigma clasts (with length parallel to S-plane), indicative of top-to-right movement sense. Note darker and lighter bands throughout the rock. j) Typical general appearance of flow banding: felsic bands are associated with aligned K-feldspar megacrysts, mafic bands develop in between.