1.Route
Dongshankou-Guandixi-Yitiaolong-Leigushan.
2.Purpo Se
(1)Observe and describe the lithology of the Fangshan pluton.
(2)Observe the primary magmatic structure in the Guandi monzodiorite unit at Dongshankou.
(3)Observe the intrusive sequence of the monzodiorite,quartz monzodiorite,pegmatite and aplite.
(4)Recognize the composition,shape,size,distribution ofi nclusions and identify the different types,such as xenolith and restite.
（5)Observe the intrusive contact between the quartz monzodiorite and the tremolite-and wollastonite-bearing marble of the Tieling Fm..Sketch the outcrop at a scale of 1:50.
(6)Observe the ductile-brittle deformation in the margin of the Fangshan pluton at Chechang village.
3.Main stops
Stop 1 The primarys tructures(magmatic structure)in the marginal belt of the Fangshan pluton
Location:The first quarry north of Dongshankou,south of a 150-m-high hill.
Content:The lithology of the quartz diorite and its primary structures.
Description:Monzodiorite crops out north of Guandi village,and is a adjacent to quartz monzodiorite farther north.The monzodiorite is dark gray,fine grained and has a weakly banded structure.It consists mainly of plagioclase,orthoclase and quartz.The plagioclase forms white laths,0.5-2mm long,and makes up about 50%-55% of the rock.Pale pink orthoclase forms broad subhedral laths that make up about 10%-13%.Colorless,anhedral quartz grains,about 2mm across,make up～10%.The other minerals include～10% amphibole，～8% biotite and less than 5% accessory minerale.
A weak flow structure is defined by the orientation of dark inclusions.The dip-direction and angle of the flow plane is 43°∠69°，and the flow line is 310°∠61°.Three sets of joints are present.The primary joints have a dip-direction and angle of 30°－50°and 80°－90°，respectively,whereas longitudinal joints have a dip direction of 120°－136°and an angle of 80°－86°.Transverse joints have a dip directionof100°－110°and adipof3。－10°.Many of the joints also contain veins.The joints are about 20 cm apart.
Stop 2 Multi≠stage intrusive relationship in western Guandi viIlage
Location:At the roadside on west side of the canal southwest of Guandi village.
Content:The multiple intrusive sequences of the pluton and dike.
Description:At this stop,the flow plane of the quartz monzodiorite is parallel to the contact,which cuts through the monzodiorite.A chilled border can be observed in the margin of the quartz monzodiotite.The quartz monzodiorite and monzodiorite are intruded by late dikes with crosscutting relationships.(Fig.3-9).The sequence of dikes from early to late is monzodiorite,quartz monzodiorite,mega-grained arizonite and granitic-aplite.
Stop 3 The nature of the contact between porphyritic and non-porphyritic rocks in the Fangshan pluton,Leigushan
Location:About 110 m south of Hill 126.9,in the saddle at Leigushan.
Content:The boundary between medium-grained porphyritic quartz monzodiotite and nonporphyritic,medium-grained quartz monzondiotite.
Description:North of this point,the medium-grained,porphyritic quartz monzodiotite has a granular texture,blocky structure,and contains about 20 modal% phenocrysts and 80% matrix.The phenocrysts are mostly plagioclase and K-feldspar laths,5-10 mm long,and these are set in a mediumgrained(2-5 mm)groundmass of quartz and feldspar.The plagioclase makes up 40 modal%within the whole rock and forms white,tabular to lath-shaped crystals(5mm×8 mm)with polysynthetic twinning.Epidote occurs on the surface of the grains.Subhedral,tabular crystals of K-feldspar make up about 25 modal％,and these grains show good Carlsbad twinning.Biotite makes up about～5 modal%and occurs both in the groundmass and as inclusions in the K-feldspar.The quartz makes up about 13 modal%,and grayish in color and occurs as unusual rod-shaped crystals about 3 mm long that are interstitial to the other minerals.It is reco gnizable by its color,greasy luster and co ncho idal fracture.Mafic minerals include about～15 modal%hornblende.The biotite forms small(3mm×3 mm),black,platy grains with a pearly luster,whereas the hornblende occurs as greenish,elongate grains abo ut 1 mm×5 mm.Accesso ry minerals make up～2 modal%and consist chiefly of black magnetite.
Fig.3-9 Sketch map shows the intrusive relationships in this outcrop
1—Quartz monzodiorite;2—Dark monzodiorite;3—Coarse arizonite;4—Granitic-aplite;5—Chilled bordero ft he quartz monzodiorite
South of this stop,the quart Z monzod iotite is also medium grained and has a blocky structure but lacks phenocrysts.The boundary between the two lithologies is gradational with a gradual decrease in the percent of phenocrysts and the abundance of quartz and an increase in the percent of mafic minerals from the porphyritic to non-porphyritic variety.
Stop 4 Inclusions in the margin of the pluton
Location:between hills 125m and 127.2m at the north-south bend in the valley.
Content:Medium-grained quart Z monzodiorite with inclusions.
Description:At this locality the quartz monzodiorite has numerous inclusions of different composition.The monzodiorite has a mottled appearance because of the abundance of xenoliths,some of which have been partly assimilated.
The xenoliths consist of amphibolite,amphibole plagioclase gneiss,hornblendite,metamorphosed quartz sandstone,limestone,and skarn with abundant epidote.They are subangular to round and are mostly about 20cm×12 cm across.The different xenoliths have reacted with the host quartz monzodiorite in different ways.For example,those with abundant mafic minerals are little resorbed and have relatively sharp contacts.In contrast,xenoliths of limestone and skarn,which are easier to assimilate,have gradual or transitional boundaries with the host rock.At this stop,the monzodiorite was intruded by the quartz monzodiorite,and both of them are intruded by a later felsic dike.
Stop 5 Weathering of the quartz monzodiorite in northern Guandi
Location:Hill 127.2m.
Content:Weathering patterns in the quartz monzodi Orite.
Descripti On:Observe the weathering in the rock,and interested geomorphologic laudscape,for example,mushroom-like,toad-like and elephant-like forms.
Think and discuss the origin(variable possible)of these geological phenomena.
Stop 6 The contact between the Fangshan pluton and the wall rock at Yitiaolong
Point A:The contact belt between the pluton and the wall rock.
Location:Northwest hillside at Yitiaolong(Fig.3-10)
Content:Here the quartz monzodiorite intrudes the Tieling Fm..The contact is marked by a skarn composed of wollastonite and tremolite marble.
Fig.3-10 Intrusive relationship between the Fangshan pluton and the Tieling Fm.in the northwest hillside at Yitiaolong
Description:To the north of this point,there is medium-to coarse-grained(3-5 mm)quartz monzodiorite composed of plagioclase,K-feldspar,quartz,amphibole and biotite.The rock is heavily weathered.To the south,there is banded dolomite and limestone of the Tieling Fm.,and the quartz monzodiorite forms apophyses and sills in the Tieling Fm.(Fig.3-9).Contact metamorphism at the boundary produced a skarn composed of yellowish,coarse-grained wollastonite and tremolite marble.
Stop 7 Contact metamorphic rock at Yangshigou
Purpose
1)Observe and recognize rocks of the Archean Guandi complex.
2)Observe the contact metamorphic aureole developed in the carbonate rocks of the Wumishan Fm.,and the clastic rocks of the Changlongshan Fm..
3)Compare the Mesoproterozoic and Neoproterozoic strata in the Bajiaozai-Shuanmazhuang section,paying key attention to the rock types and thickness of the strata.
Location A:150 m southeast of Guandi village,north of sheepfold at the base of the escarpment.
Content:Rocks of the Guandi Complex.
Description:The metamorphic Guandi core complex crops out here and is composed of biotite plagiogneiss,amphibole plagiogneiss,amphibolite and biotite amphibole granulite,all of which indicate medium-to high-grade metamorphism and migmatization.The biotite plagiogneiss is pale gray on weathered surfaces,and has a granoblastic texture and gneissic structure.It consists of about 55-60 modal%gray,platy plagioclase(2mm×3mm）,30-35 modal%granular quartz(2mm× 2mm）,and about 5 modal%black,platy biotite(2mm×2mm).The gneisses are intruded by thread-like felsic veins,a large pegmatitic vein and an amphibole dike.The rock appears to have been migmatized.The strike and dip angles of the gneissosity are 152°∠62°，and the strike-dip angles of the pegmatitic vein are 95°∠18°.
Most of the Guandi complex has been overprinted by dynamic metamorphism to form blastomylonite,mylonite and cataclastic mylonite,all of which have a banded structure with the bands being about 0.2～1mm thick.
Location B:130 m south of Yangshigou.
Content:Mesoproterozoic rocks of the Wumishan Fm..
Description:North of this stop,Quaternary sediments overlie the basement rocks,and the Wumishan Fm.(Jxw)crops out to the south.Up to this point,the boundary between the Wumishan Fm.and the Archean Guandi complex is covered by Quaternary sediments.To the east of this poin in the Zhoujiapo area,a detachment fault separates the Wumishan Fm.from the Archean rocks.
The Wumishan Fm.consists of grayish,thick-bedded wollastonite and tremolite marble,with a fibrous,granoblastic texture and blocky structure.The tremolite forms grayish,needle-shaped or white,slaty crystals,about 1.5mm×3 mm in size,with a vitreous luster.The wollastonite is relatively soft and can be scratched with a knife.Both the tremolite and wollasto nite occur in siliceous clusters.The layer has strike-dip angles of 170°and 55°.
Location C:Northwest of Luotuoshan at the escarpment in Yangshigou.
Content:The boundary between the Neoproterozoic Xiamaling Fm.and the Mesoproterozoic Tieling Fm.,and zonation in thermally metamorpho sed rocks.
Description:To the north of this stop,the Meso proterozoic Tieling Fm.,composed of marble and dolomitic limestone,crops out.To the south,the Neoproterozoic Xiamaling Fm.is composed of zoned contact metamorphic rocks.From north to south,the section includes:
(1)Gray,garnet mica schist with a porphyroblastic to lepidoblastic texture and a schistose structure.The porphyroblasts are reddish garnet,0.5～0.8mm across,with numerous inclusions and some banding.The schistose structure is formed by oriented grains of mica and quartz.
(2)Gray,andalusite garnet mica schist with a porphyroblastic to lepidoblastic texture and a schistose structure.The porphyroblasts are andalusite,which occurs as grayish,columnar crystals,about 2mm×10mm,with their long axes parallel to the schistosity.The andalusite is unevenly distributed and is concentrated in layers.The garnet crystals are about 0.5 mm across and are less abundant than in zone①.The strike-dip angles of this layer are 205°and 46°.
(3)Greenish,andalusite mica schist,with a porphyroblastic to lepidoblastic texture and a schistose structure.The porphyroblasts are gray andalusite,which forms long,columnar crystals about 2×10mm in size.Some of the crystals have a silky luster,which may be the result of sericitization.The garnet gradually decreases and finally disappears.
Stop 8 The northwest margin of the Fangshan pluton,east of Che Chang village
Location:The southwestern part of the arcuate deformed zone at the western margin of the Fangshan pluton.
Content:Observe ductile-brittle deformation in the margin of the Fangshan pluton.
Description:Westward from the centre of the pluton,the penetrative foliation becomes gradually more obvious and the quartz monzodiorite changes into gneissic quartz monzodiorite with a variety of angular and disc-shaped xenoliths.The aspect ratio of the xenoliths is 10-50 at the centre of the zone.Two sets of ductile shear zones can be seen in this zone,but commonly only one of them occurs in a given outcrop.Within shear zones the massive monzodiorite,xenoliths and aplite veins are completely mylonitized.The bisectrix of the obtuse angle of the conjugate shear zones is parallel to the compression direction and coincides in orientation with the normal of the earlier foliation(gneissosity).At this stop,the gneissosity is vertical and strikes to the north.One set of shear zones is also vertical,and strikes to the NNE with a dextral strike-slip sense,whereas the other set strikes NNW and has a sinistral strike-slip sense.
In the upper part of the outcrop,a laterb rittle faultc uts the gneissosity at a high angle.These structures indicate a pulsation of magmatism and ballooning-type plutonism.
4.Brief Summary
In this traverse,we have observed the monzodiorite and quartz monzodiorite Of the Fangshan pluton,and noted the lithol ogy,mineralogy,texture and structure of the rocks.We have also observed the contact metamorphic zone on the south margin of the pluton and the Subsolid-state deformation of the rocks.
5.Questions for discussion
(1)What is the evidence for two phases in the Fangshan pluton and how the boundary could be distinguished?
(2)How can one identify the inclusions?
(3)What evidence can be used to determine the intrusive sequence of the plutonic rocks and the dike?