Download Guidebook as .pdf (29.1 Mb) - Carolina Geological Society

More documents

Recommendations

Info

MAFIC MINERAL CRYSTALLIZATION IN SOUTH CAROLINA DIABASE Figure 4C. Shoals Junction diabase. Samples are, clockwise from upper left, SJD-A; backscattered electron image showing euhedral olivine microphenocryst (center) and plagioclase laths (dark) in SJD-A; SJD-B and SJD-C. Progression is from microporphyritic to intergranular texture. 63
Richard D. Warner, Nicholas B. Kidd, David S. Snipes and Jeffrey C. Steiner Figure 5B. Pyroxene quadrilateral plot showing composition of pyroxene in Easley diabase. Figure 5A. Histograms showing composition of olivine in Easley diabase. Solid bars indicate composition of cores and striped bars give compositions of rims and/or groundmass olivines. (Figs. 5-7). EFFECT OF COOLING RATE In both the Easley and Edgemoor diabases there is a progressive change in pyroxene and olivine compositions related to distance from the contact. In order to clarify this relationship, we have selected for comparison the compositions of the earliest-crystallizing (i.e., most magnesian) olivine and pyroxene in each sample from the two dikes. The results are plotted in Figure 8 as a function of distance from the contact. Examination of this figure reveals that there is a decrease of 7-8% in Fo content of the earliest-crystallizing olivine between the chilled margins and centers of both dikes, and that there is a concomitant increase of 12-16% in En content of the earliest-crystallizing pyroxene. A similar relationship of decreasing %Fo and increasing %En as a function of increasing grain size (slower cooling) was reported for the Due West diabase in Abbeville County (Warner and others, 1986) and for several thick dikes in Chester and Fairfield Counties (Warner and others, 1993). Thus, it appears to be a common feature of relatively thick diabase dikes. Other aspects of the cooling rate dependence become apparent when microprobe analyses of individual calcic pyroxene crystals are compared (Table 2). The data indicate that, in addition to MgO, there is a marked increase in SiO 2 and Cr 2 0 3 in augite as one moves toward the centers of the dikes, while TiO 2 , Al 2 O 3 and FeO significantly decrease. We attribute the compositional changes observed above to varying degrees of disequilibrium crystallization. Very rapid cooling at the dike contact effectively quenched the liquidus olivine compositions, allowing little opportunity for olivine crystals to react with the magma. Pyroxene compositions were also quenched: note how closely SiO2, FeO and MgO in the Easley/Edgemoor chilled margin pyroxenes resemble the values found in the bulk rocks (Table 1). Where slower cooling prevailed, toward the centers of the dikes, there was a closer approach to equilibrium. Under these conditions, early-formed olivines had time to partially react with the melt and convert to more Fe-rich crystals. The reactions were far from complete, however, as evidenced by extensive 64
Page 3 and 4:
CAROLINA GEOLOGICAL SOCIETY 1995 OF
Page 5 and 6:
Mafic mineral crystallization in So
Page 7 and 8:
Allen J. Dennis and Others Turn rig
Page 9 and 10:
Allen J. Dennis and Others Fig 4. V
Page 11 and 12: Allen J. Dennis and Others and a nu
Page 13 and 14: Allen J. Dennis and Others Fig. 8.
Page 15 and 16: Allen J. Dennis and Others Fig. 10.
Page 17 and 18: Allen J. Dennis and Others graphic
Page 19 and 20: Allen J. Dennis and Others group) o
Page 21 and 22: Allen J. Dennis and Others includes
Page 23 and 24: Allen J. Dennis and Others grey roc
Page 25 and 26: Allen J. Dennis and Others lel to t
Page 27 and 28: CAROLINA GEOLOGICAL SOCIETY Guidebo
Page 29 and 30: GEOLOGIC CONTRASTS ACROSS THE CENTR
Page 37 and 38: GOLD DEPOSITS OF THE WEST SPRINGS A
Page 45 and 46: Allen J. Dennis and Others Figure 1
Page 47 and 48: Allen J. Dennis and Others The unna
Page 51 and 52: GEOLOGY OF THE WESTERN PART OF THE
Page 57 and 58: Richard D. Warner, Nicholas B. Kidd
Page 61: Richard D. Warner, Nicholas B. Kidd
Page 71 and 72: PETROGRAPHIC AND GEOCHEMICAL STUDY
Page 73 and 74: PETROGRAPHIC AND GEOCHEMICAL STUDY
Page 77 and 78: STRUCTURAL CONTRASTS OF THE CAROLIN
Page 85 and 86: GILES ALLARD AND JAMES WHITNEY disc
Page 87 and 88: GILES ALLARD AND JAMES WHITNEY and
Page 89 and 90: GILES ALLARD AND JAMES WHITNEY W.,
Page 91 and 92: JEFFREY E. CLIPPARD AND ROBERT B. H
Page 103: JEFFREY E. CLIPPARD AND ROBERT B. H
show all

Download Guidebook as .pdf (29.1 Mb) - Carolina Geological Society

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?