| Analcime-bearing dolomite is a rare type of rock, which usually occur in lacustrine environment recorded in limited researches. Its origin, which interpretations yielded by foreign and domestic researchers show great difference, now become a hard nut. This kind of special rock was discovered while drilling in lower part of Shahejie Formation in Tanggu area, which provides an exceptional chance for us to learn its characteristic and origin. Based on data of core, thin section, X-ray diffraction analysis (XRD), cathodoluminescence analysis(CL), scanning electron microscope analysis(SEM), energy dispersive spectrometer analysis (EDX), geochemical analysis and mircopaleontology analysis, the characteristics of rock and minerals, geochemistry and micropaleontology of the analcime-bearing dolomite were summarized. With the guide from sedimentology, petro-mineralogy, element geochemistry, isotope geology and micropaleonotogy, the provenance and paeleo-sedimentary environment were reconstructed, and the link between formation of the rock and geological processes like hydrothermal sedimentation, volcanism, weathering, microbial meditation, and evaporation was also built. With the mechanism of analcimization, dolomitization and formation of lamina association being clear, the reconstruction of the formation of analcime-bearing dolomite was finally presented. Several major ideas are listed below:Dolomite, argillaceous dolomite, dolomitic mudstone, mudstone and calcareous silt were recognized in the 77.45m long core of key Well T12C of study area. Dolomite series (dolomite+argillaceous dolmite) and mudstone series (dolomitic mudstone and mudstone) are principal component of the core, which exhibit a package form in space (dolomite series being enveloped in mudstone series). Laminated bedding is the most common feature found in different rock species. Massive bedding frequently occur in dolomite and convolute bedding usually can be observed in argillaceous dolomite.10 lithofacies associations were divided by vertical alternation of lithofacies. Laminated mudstone-laminated dolomitic mudstone-laminated argillaceous dolomite association relative with transistion from deep water sedimentation to shallow water sedimentation, laminated dolomite-massive dolomite-laminated dolomite association relative with shallow water sedimentation, laminated mudstone-massive dolomitic mudstone-laminated mudstone relative with deep water sedimentation are more common to be observed than other species.The mineralogical characteristics of dolomite series of Sha3-5 submember were fully disclosed by combination of the thin section identification, cathodoluminescene analysis, XRD analysis, SEM analysis and EDS analysis. Dolomite, analcime, quartz, feldspar (orthoclase and albite), clay minerals, pyrite and barite were identified in the rocks and dolomite, analcime, quartz and feldspar are major components. The dolomite series is actually an analcime-bearing dolomite. The abundance of dolomite can range from 8% to 40% for dolomite and from 25% to 52% for argillaceous dolomite. Dolomite mineral in the rocks is generally anhedral dolomicrite (<4μm) and a ferruginous type (ankerite) with low ordering degree, which usually occur as laminae or evenly disseminated in matrix. The abundance of analcime range from 25%-65% for dolomite and 26%-38% for argillaceous dolomite. Mircocrystalline (<4μm) analcime mineral, a silica-poor type of Group C reported by DS Coombs and T Whetten, is usually anhedral and generally occur as lamina or matrix constituent, while coarser analcime, a silica-rich type of Group A, is generally euhedral and can be found in vein or vug. The abundance of quartz is from 6% to 17% for dolomite and from 10% to 14% for argillaceous dolomite. The abundance of feldspar is from 9%-18% for dolomite and from 6% to 17% for argillaceous dolomite.These two silt-sized sub-angle-sub-rounded minerals usually occur together as lamina or disseminated in the matrix. The abundance of clay minerals varies from 3% to 13% for dolomite and 7% to 11% for argillaceous dolomite and is illite-dominated, but some mixed-layer minerals of illite and smectite can also observed. Pyrite in the rocks have various occurrence like grain, lamina, stripe or patch.5 laminae association in laminated dolomite series can be divided by alternation of different lamina. Mud-rich laminae-(kerogen-rich laminae)-analcime-rich laminae association and ankerite-rich laminae-(silt-rich laminae)-analcime-rich laminae association are two common types.The geochemical features of rocks of Sha3-5 submember were summarized by element geochemistry analysis, C-O isotope analysis, and Sr isotope analysis. Compared with Upper Continental Crust (UCC), the major element of dolomite series were enriched in Na, Mg, P, and Ca and slightly depleted in Si, Ti, Fe, and Mn; mudstone series were usually enriched in Mg, P, and Ca and depleted in Na, Si and Mn. The trace element of dolomite series were enriched in B and Sr, and depleted in Sc, Cr, Co, Cu, Mo, and Ho. The enrichment and depletion of Ni and U in dolomite series varies considerably, and the content of Ba is similar to that of UCC. The trace element of mudstone series shows similar trend of enrichment and depletion with dolomite series, but they are more enriched in B. The EE of dolomite series and mudstone series are higher than that of UCC (146.37×10-6) and little lower than PAAS (184.77×10-6). Both dolomite series and mudstone series are enriched in light rare earth element and depleted in heavy rare earth element, and light rare earth element tend to accumulated in mudstone series. The REE pattern of the rocks exhibit moderate negative Eu anomaly and slight negative Ce anomaly. The oxygen isotope composition of dolomite are low. The δ13O VPDB in dolomite series is from -7%o to-1.7‰ with an average of -4.7‰; the δ13O VPDB in mudstone series is from 8.6%o to-4.5‰ with an average of-6.58‰. Coarser dolomite bear lower oxygen isotope composition than finer dolomite. The carbon isotope composition of dolomite are generally high except one sample. The δI3C VPDB in dolomite series is from 4.6%o to 8.7%o with an average of 6‰; the δ13C VPDB in mudstone series is from -2.5‰ to 5.4%o with an average of -2.1‰.These ranges are different from that of dolomite in upper part of Sha4 Formation in Dongying depression and Sha3 formation in Liaodong Bay Basin. The 87Sr/86Sr ration of the rocks range from 0.70863784 to 0.70896268 with an average of 0.708865048, and show less difference in dolomite series and mudstone series. However, the 87Sr/86Sr of the rocks are lower than that of dolomite in Sha3 Formation of Well wang35 of Qijiawu area, Qikou depression, and limestone and dolomite of lower part of Sha3 Formation of Chezhen and Zhanhua depression.With the data of the micropalaeontology analysis, micropalaeontology feature of Sha3-5 submember can be summarized. Angiosperm pollen (47.2%-57.0%, av.=52.1%) dominate, following by gymnosperm pollen (40.0%-49.6%, av.=44.5) and fern spores (2.8%-4.6%, av. =44.5). Quercoidites (16.4%-19.4%) take up most of angiosperm pollen, Pinaceae with dual airbags predominate in gymnosperm pollen. Polypodiaceaesporites is the most common fern spore component. The pollen association is close to the Quercoidites microhenrici-Ulmipollenites type found in Sha3 Formation of Bohai Bay Basin. Fossil algae species are more monotonous, and typical sub-Bohai algae and Bohai algae do not appear. Acritarchs occur frequently, and the pyrrhophyta and chlorophyta are similar.Based on Eu anomaly and Y/Ho ratio (dolomite:25.4-29.7) close to upper crust(27.5), we proposed that the formation of dolomite series was accompanied by a lot of terrigenous material supply. The average Al2O3/TiO2 ration is 25.36 for dolomite series and 24.8 for mudstone series, which indicate felsic source (21-70). K-Rb, La/Th-Hf bivariate diagram also suggest felsic source. Th/Sc-Zr/Sc bivariate diagram suggest the clasts is not a recycling product. With the evidence listed above, Yanshan fold and thrust belt was redertermined as the major provenance of clastic component rather than Cangxian uplift.Pollen vegetation type, temperature zone and humidity zone were restored by pollen association, and needle-broad-leaved mixed forest types, northern subtropical type and semi-arid-semi-humid type can be generalized. The equilibrium temperature calculated from oxygen isotope composition is from 52.5℃ to 81.3℃ with an average of 65.9℃ for coarser dolomite, and; is from 34.6℃ to 56.5℃ with an average of 49.3℃ for finer dolomite, indicating an arid environment.The conclusion of saline water type during sedimentation can also be drawn by combination of analcime enrichment, m values (dolomite series:av.= 31.21, mudstone series:av.=22.66), Sr/ Ba ratio (dolomite series:av.= 1.2, mudstone series:av.=1.07), Z value (dolomite series:av.= 137.2, mudstone series:av.=128.3) and the algae association. Besides, classic B methods is no longer fit for the paleosalinity reconstruction in Sha3-5 submember. The oxic condition during sedimentation can also deduced from U/Th (dolomite series:av.=0.32, mudstone series:av. =0.29), authigenic U (dolomite series:av.= 0.96, mudstone series:av.=0.89) and Ce negative anomaly.Volcanism was proposed to has no close connection with the formation of analcime for the following reason:(1) the REE pattern of the rocks of Sha3-5 submember is different from that of basalt of Sha3 member, but take similarity with that of granite formed during Yanshan period; (2) analcime seems to be a major carrier for REE by element correlation analysis; (3) mafic glass is unable to provide enough REE; (4) no relict of volcanic glass texture can be observed in thin sections;(4) silt-rich lamina contains no evident glass composition other than allogenic particles (quartz and feldspar) and analcime-rich lamina accumulates only analcime rather than other mineral clasts; (5) analcime have lower Si/Al ratio (2.09-2.36). With the existence of smectite and globular shape of analcme, we believed that the analcime is likely to be transformed from clay minerals (smectie) formed during weathering, but we also do not exclude the possibility that analcime might altered from gel precursor. In addition, analcime in vein and vug were considered to be a kind of hydrothermal product by its Si/Al ration and occurrence.The microbial activity might involve in the formation of dolomite mineral in argillaceous dolomite and dolomitic mudstone for the following reasons:(1) the occurrence of microbial mats; (2)near-round crystal shape of dolomite; (3) higher δ13C (7.2‰-8.7‰) in argillaceous dolomite and relative considerable changes of δ13C (-2.5％-5.4‰) in dolomitic mudstone. However, the type and mechanism of microbes that take part in the formation of dolomite might be different. The paleosalinity during sedimentation is considered to be control by temperature, deducing from semi-arid-ancient semi-humid climate, good relationship between water temperature and paleosalinity and chemical salts dissolved pores observed in the massive dolomite. The sabuka, brine reflux and dorag dolomitization models are not applicable to the formation of dolomite in the study area for the following features (1) finer crystal, usually less than 4um; (2) lack of gypsum nodule or layer; (3) lack of mold pore; (4) early isopachous calcite microspar cement;(5) lack of vadose cement. Based on close relationship between analcime and dolomite, the precursor of analcime and previous studies, we proposed that the Mg2+ released from smectite during analcimization will incorporate into the early-formed aragonite/Mg-calcite and bring the dolomite out. By space distribution of dolomite and analcme and idea of Mg2+ provided by analcimization, we thought that analcme and dolomite might formed simultaneously, and analcime also might postdate dolomite.Laminae in Sha3-5 submember were consider to be a product of climate alteration based on the idea that climate change have direct impact on the prevalent rhythmic laminae founded in ancient-modern lake sediments. During the period of higher lake level, mud-rich laminae formed in humid period while analcime-rich laminae formed in arid period. During the period of lower lake level, analcime-rich laminae formed in humid period while ankerite-rich laminae formed in arid period. Silt-rich laminae formed when floods occured. Kerogen-rich lamina can formed when the water condition was favored. The massive bedding is associated with long period of evaporation result from drought climate.In all, the formation of analcime-bearing dolomite of Sha3-5 member is the product of combination of weathering, evaporation and a chain of diagenesis. Weathering provide sufficient clay minerals (smectite).Evaporation bring in high salinity and precipitation of aragonite/Mg-calcite as well as analcimization. The Mg2+ released from analcimization, then, result in dolomitization. The incorporation of Fe2+ from clay minerals into dolomite later yield ankerite. |
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