There are subvolcanic rocks in Yixian-Beipiao area in the late eruption period of Yixian volcanic cycle. Subvolcanic rocks mainly include olivine basaltic porphyrite, olivine basaltic andesite porphyrite, andesite porphyrite and so on.
Olivine basaltic porphyrite and olivine basaltic andesite porphyrite: the most developed. The rock appearance characteristics of the two are very similar and difficult to distinguish, which is often accompanied by a gradual transition trend. In addition, there are still a few alkaline olivine basaltic porphyrites in Xiashichang area of Yixian County (plate 35-2). Basaltic porphyrite mainly exists in the form of oval, irregular and discontinuous banded and small rocks, dikes (veins), bedrock (plate 32- 1 2), mushroom-like rock necks and other small rock masses with different surface sizes, which are distributed in the first sub-cycle (including Sihetun, Shangyuan Town, Beipiao) and the northern half of the second and third sub-cycles. The mineral characteristics of rocks are similar to those of olivine basalt and olivine basalt andesite. The rocks are grayish black, dense, massive and porphyritic. Olivine basaltic porphyrite (15% ~ 20%) is slightly more than olivine basaltic andesite porphyrite (10% ~ 15%), which is mainly composed of authigenic, semi-authigenic olivine and a small amount. Olivine is mainly composed of pseudoamphibole, chlorite, serpentine, talc, chalcedony and separated iron. Olivine residual crystals account for 30% ~ 80%, and the interference colors are secondary blue-green, orange-red, reddish purple. The angle of 2v (+/-) optical axis is large, and a few of them are close to 90. Combined with electron probe analysis, it should belong to forsterite-forsterite. The matrix is in the form of interwoven lath, which is composed of slab of Gengzhong and Middle Labrador (No29-50-67), iron-magnesium mineral particles and carbonate.
Andesite porphyrite: it can be divided into two types: less-spotted structure and multi-spotted structure. The former is mainly distributed in the basaltic andesite-andesite series east of Zhuanchengzi in the main period (Ⅱ), and the area from Xialing to Xiaoweijiagou. The dike length is 300 ~ 1000 m, and part of the dike length is 1500 m ... The andesite porphyrite distributed in the late (third) cycle is well developed, and irregular small rock masses (plants) with different sizes in the northeast cut through andesite, Dakangbao and Zhujiagou sediments, which can be divided into coarse spots and multi-spots.
The andesite porphyrite is in the form of dike group: grayish green, grayish black, dense and massive, with a few spots, which account for about 3% ~ 5% and 65,438+00% locally. It is composed of tabular plagioclase (No.25 ~ No.27), a few intermediate feldspar with banded structure (No.35 ~ No.37), olivine and clinopyroxene. Olivine has been replaced by fibrous scale mica, serpentine, granular chronology and iron aggregate, showing its illusion. Clinopyroxene is relatively fresh, self-columnar, with a large optical axis angle of 2V(+), interference color secondary blue-green, orange-red, and C∧Ng=45. It has simple twin crystals, which are ordinary pyroxene. Matrix interweaving (Figure 35-5), a small amount of glass crystals interweaving.
The andesite porphyry with medium-coarse porphyry: its lithology is sometimes indistinguishable from that of the same type andesite, and it is gray-black with medium-coarse porphyry structure, with porphyry crystals accounting for about 65,438+05%, mainly consisting of 2 ~ 4 mm slab-topped feldspar (No.28 ~ 40), a small amount of olivine (carbonized) and clinopyroxene (ordinary pyroxene) C \ matrix.
Porphyry andesite porphyry: in the contact zone between the main period and the third sub-cycle, there is a mushroom-like roof (such as 302.2 highland in the northeast of Sanbailong). The rock is gray-black, with massive structure and developed local pore structure (accounting for 60% ~ 70%). Porphyry structure, containing about 35% ~ 40% of porphyritic crystals, mainly composed of automorphic-semi-automorphic tabular plagioclase (No27) (plate 35-6), with particle size of 2 ~ 5 mm, with Casbas twins, fine flaky twins and Karna twins. The matrix is interwoven and consists of plagioclase and a small amount of clinopyroxene, iron minerals and carbonate.
Ying 'an porphyry and rhyolite porphyry: undeveloped, mainly found in the northwest corner, northeast corner and south of the survey area in the form of rock wall and small rock mass, and occasionally found in Huanghuashan layer in the north of Zaocishan and Yixian upper cycle.
(2) quenching (pillow) lava
From 200/kloc-0 to 2002, the volcanic rocks in this project were discovered and confirmed for the first time. Quenched basalt (basaltic andesite), quenched pillow basalt (basaltic andesite) and giant olivine-bearing pillow andesite formed by rapid cooling of underwater environment in continental lake basin in late Mesozoic have never been seen before in this area, which is of great significance to further study the volcanism, temporal and spatial evolution law and volcanic rock series characteristics of Yixian cycle.
1. distribution and age of quenched (pillow-shaped) lava
According to the relative production relationship between the quenched basaltic-basaltic andesite pillow lava and the rare fossil horizon of Yixian Formation in this area, it can be divided into three stages:
Phase I: It is exposed in the south of the observation post of Sihetun Fossil Field and on the top of Jianshan Rare Fossil Deposit, with a length of about 100 m and a width of about 30 m. The pillow lava of quenched basalt (basaltic andesite) is distributed in the middle and upper part of Zhuchengzi sedimentary layer (the horizon is equivalent to Jianshan Rare Fossil Layer in Sihetun District), and it is 7.5 km north. Similar quenched lava was found in continental basaltic lava in northern Hebei (Xia Guoli, 1993).
Phase II: The giant pillow andesite (plate 34-6) distributed on the southwest coast of Xu Jiatun (with normal detritus of sandstone and siltstone) is called Phase II, which belongs to the product of the middle and late main period of Yixian volcanic cycle (second subcycle).
The third stage: pillow basalt exposed in the north of Taoxigou and the upper part of Dakangbao sedimentary layer in yi county. Its occurrence horizon is higher than the rare fossil layer in Jianshangou and the brick sublayer in Sihetun, which may belong to the same product of Dakangbao layer and belong to the product of late yi county cycle (the third sub-cycle), so it is called the third stage.
2. Characteristics of quenched lava-pillow lava
The quenched basalt (basaltic andesite) (pillow lava) exposed in the south of Sihetun Fossil Observation Post and Jinjiagou-Dakangbao area is very similar to its homogeneous pillow lava in macro and micro structure and structural characteristics: macroscopically, pillow lava is mostly produced in groups and belts, often with quenched (pillow) basalt (basalt andesite) (Sheet 30) pillow basalt (basalt andesite). The size of the pillow body is generally 1 ~ 3m in length, some are more than 4 m in width, generally 1 ~ 1.5m, and some are 2 ~ 3 m (Figures 34-2 and 4). There is a layer of gray-green dense quenched (solidified) edge around the pillow body shell, and the thickness is10 ~ 20cm (30 ~ 40cm locally). Except for a few pincushions, most pincushions are composed of self-crushed (block 34-5) blocks and breccia (generally 5-20 cm) formed by quenching, which is different from homogeneous quenched lava in whether it has pincushions. The pillow body is partially filled with tuffaceous sedimentary rocks except for homogeneous quenched lava debris. The rocks are gray-black, black and brittle, and there are few almond-shaped structures and porphyritic structures, with porphyritic crystals accounting for about 65,438+00% ~ 30%, which are mainly composed of olivine (mainly olivine), clinopyroxene (mainly augite, rare diopside and metamorphic pyroxene) and plagioclase (intermediate-pyroxene), in which olivine has been completely altered to serpentine. The matrix and glass crystals are intertwined.
Occurred on the steep bank wall at the southwest end of Xujiatun, the pillow andesite is characterized by massive irregularity, ellipsoid and tongue-shaped pillow, rich in almonds, 2-4 m in diameter and 5-6 m locally, with clear pillow structure, consisting of a gray-green annular shell rich in medium and small almonds, a transition zone and a yellow-gray pillow core with a small amount of coarse almonds (Figure 34-6). Microscopically, it can be seen that almond stone has a tendency from more to less, from small to large, from complex to simple in morphology and structure, and its components are from siliceous, montmorillonite-siliceous, montmorillonite-carbonate to montmorillonite. The pillow body has a porphyritic structure, with porphyritic crystals accounting for 3% ~ 5%, which is composed of authigenic and semi-authigenic olivine illusion (granular syenite, mica and serpentine aggregate) and clinopyroxene. The matrix and glass crystals are intertwined.
In petrochemical characteristics, the crushed pillow basalt and basaltic andesite in Sihetun and Yaomashangou belong to basaltic trachyandesite and high-K calc-alkaline rock series.
In addition, according to the report of Zhang Lidong et al. (200 1), pillow lava is also exposed in Wudaigou and other places.
(3) honeycomb explosive (concealed explosion) aggregate breccia tube
Honeycomb-like explosion (cryptoexplosion) basalt andesite aggregate breccia tube was first discovered and confirmed by this special group in Beipiao-Yixian area of western Liaoning at 200 1. Its discovery is of special significance to objectively understand the volcanism of Yixian Formation in this area and correctly define the volcanic stratigraphic sequence of Yixian Formation.
1. distribution characteristics of breccia tube of honeycomb explosive (concealed explosion) group
Honeycomb explosion (cryptoexplosion) aggregate breccia tube is bedded, veined, tubular andesite-basaltic andesite aggregate-breccia (molten rock), which is honeycombed in space, and its surface morphology is distributed in Yixian volcanic cycle and Sihetun volcanic structural depression (Figure 3-4-2), especially in the main period of Yixian cycle (the second sub-cycle) in Yixian area. The dense areas are mostly distributed in the andesite and basaltic andesite series in a strip shape, with dense tubes, columns and dark tubes, which run through basalt, basaltic andesite and andesite lava (plates 32-5 and 6) and its early clastic strata; Most of them are round and oval in plane, with different sizes (mainly small), ranging from several meters to more than ten meters in diameter, and some of them can reach 100 ~ 200 meters. They are produced in the form of eruption-eruption craters, which often form dense craters or constitute volcanic eruption centers (see Yixian standard stratigraphic profile and geological sketch of Yixian standard stratigraphic profile). Among them, the distance between the craters in the densest area is only a few meters to more than ten meters or directly contacts. There are at least hundreds of such craters with a small scale, and they are countless. They are also distributed in different degrees in the early stage of Yixian cycle (the first subcycle), the upper stage and the late stage (the third subcycle). In Sihetun area, this basaltic andesite-andesite aggregate-breccia lava mainly occurs in layers, a few in tubes, tubes or veins, and most in colors. Its volcanic activity intensity and product development degree are also relatively weak. In general, it is distributed alone or at intervals of tens of meters, and it is also distributed in bands in andesite and basaltic andesite series.
Figure 3-4-2 Occurrence Characteristics of Honeycomb Explosion (Concealed Explosion) Breccia Tube
Specifically, it can be divided into three states:
1) red, crimson, grayish purple and grayish green veined, tubular pore almond-shaped intermediate-basic and neutral aggregate breccia (or breccia aggregate) lava. Honeycomb or sporadic distribution, found in the south, north and Daweijiagou area of Brickyard Town, yi county;
2) Brick-red layered medium-basic and neutral lava, with vein lava on the basis of layered lava. Generally accompanied by the first output, as seen on both sides of the northeast road of Brick City Road;
3) Gray-green pores, almond-shaped intermediate basic and neutral lava (tube? ), round or blocky, several meters in diameter, pores and almond-shaped shape become smaller from the center to the edge, with grayish white condensed edges; The intermediate-basic and neutral lava between the invading cylinders rotates along the condensation edge and has corresponding cleavage structure, or the cylinders are in direct contact. Generally, it is produced in the form of honeycombs and strips, and no sporadic distribution is found. It is found in Xiaxujiagou-Jiazishan area in yi county.
In Donggou, Maojiadian, Yixian County, it can be seen that gray-green vein andesite aggregate breccia lava is penetrated by brick-red vein andesite aggregate breccia lava. It shows that the formation time of the two may be slightly different.
2. Rock characteristics of tubular basaltic andesite-andesite assemblage breccia (lava)
Due to the complexity of volcanic action mode, the difference of action intensity and frequency, that is, from concealed explosion to explosion or explosion, the products formed by basalt andesite-andesite aggregate breccia lava often have no obvious sorting and rhythm, and most of them are "mixed accumulation". Even in the crater or eruption center of the same rock pipe, we can see not only angular and sub-angular volcanic blocks and breccias of different sizes, but also layered and massive slag pumice-like homogeneous lava with dense pore structure (Figure 32-4), sometimes "massive" clastic lava formed by melting or weathering, and occasionally spherical, ellipsoidal or rope-like explosive phase products. Seen from Sigou, Yixian County-289.0 Highland. In addition to the above-mentioned rock characteristics, the lava of andesite in the 256.3 highland of Sigou volcanic eruption center intruded into the lava column with a height of more than 6 m (Figure 32-3). In Dizang Temple-Xiazhoujiagou, Jingjiatun, Xiaofangshengou Dongshan and other places, there are still concealed homogeneous "cryptoexplosive block breccia tubes". In a word, this kind of andesite-basaltic andesite breccia lava is a special kind of rock, which has the characteristics of explosive phase, cryptoexplosive breccia phase and equal intrusion.
Another feature of this kind of rocks is that no matter how different their modes of occurrence are, their appearance characteristics are very similar, with special crimson, purple gray and brick red, all of which belong to andesite or basaltic andesite. Near the crater, there are mostly giant coarse-grained fused masses, breccia and conglomerate-breccia lava (Figure 32-6), and occasionally there are massive volcanic bombs.
Microscopically, the rock has a porous almond-shaped structure. The amygdala is mainly lenticular, spherical and ellipsoidal, with a small amount of worm-like and irregular (plates 36-5 and 6), generally concentric rings (with 3-8 rings), shell-like and fibrous radial structure, accounting for about 25%-40%, and in some cases accounting for 50%-70%, mainly composed of montmorillonite and montmorillonite. Porphyry structure: Porphyry crystals account for about 3% ~ 10%, and are mainly composed of 0.5 ~ 2 m self-shaped and semi-self-shaped columnar olivine, most of which are serpentine, biotite, amphibole, hypoamphibole, timely and so on. And present its illusion. The matrix is interwoven structure and glass-crystal interwoven structure, which is composed of banded and acicular feldspar (No.30 ~ No.45) and a small amount of monotypic pyroxene (ordinary pyroxene) and biotite. Monocline is self-shaped, semi-self-shaped columnar grain, secondary orange interference color, simple twin, with a large value of 2V(+) and c ∧ ng = 40 ~ 42. In Houyanzigou, this kind of rock contains more authigenic and semi-authigenic columnar granular apatite.
Basalt andesite-andesite (fused) breccia breccia (fused) rocks are mostly located in basaltic coarse andesite areas, and some of them are located in basalt and basaltic andesite areas, still belonging to high potassium calcium alkaline series. According to the petrochemical characteristics, its original magma and volcanic rock series of Yixian Formation come from the same magma source. At the end of magmatic evolution, a set of basaltic andesite-andesite magma with high alkali, high potassium and olivine was formed by crystallization, which erupted on a large scale along volcanic structural depressions in favorable regional fault tectonic environment.
The basaltic andesite-andesite (fused) block-breccia in the volcanic rock series of Yixian formation in western Liaoning has not been reported so far, and it was treated as a volcanic rock stratum by predecessors, so the thickness of volcanic rock series of Yixian volcanic cycle is often artificially increased. If isotopic and magnetic dating samples are taken, the dating of volcanic rocks in Yixian Formation is also based on unscientific basis. At present, it is preliminarily found that there is a set of bedding and vein-like aggregate-breccia (molten) rock (tube) in the middle and late volcanic cycle of Yixian Formation, which is formed by intrusion-intrusion, cryptoexplosion and overflow explosion, and has the characteristics of explosive phase, overflow phase, cryptoexplosion breccia phase and even intrusion. It should be separated from the volcanic strata of Yixian Formation, and its exposed area should not be regarded as the thickness of the strata. Therefore, its discovery objectively determines the thickness of the volcanic rock series of Yixian Formation, further clarifies the rock assemblage characteristics of Yixian fire cycle, and provides an important basis for further studying the volcanism and age of Yixian Formation in a broad sense in western Liaoning.