摘要:

Commonwealth Edison recently drilled three engineering test holes in northwestern Illinois. The three holes (UPH 1, 2, and 3) were 0.6, 1.6, and 1.6 km deep, respectively, and the latter two penetrated nearly 1 km of Precambrian granitic basement. In 1980, continuous core from all three holes and access to UPH 3 were made available for scientific investigations through the Illinois Deep Hole Project. The drill hole and cores provided an excellent opportunity for “piggyback” scientific investigations because (1) the basement geology of the region was poorly known, (2) a large vertical depth of continuous core was available, and (3) one of the holes was available for testing where corresponding core is available. The principal study of these holes was an integrated investigation of surface and in‐hole experiments, plus detailed petrologic, geochemical, and physical property studies of the core samples. The combined results of these studies contributed significatnly to our understanding of the regional geology of the Precambrian basement, the regional geophysics and tectonics, and the correlation between geophysical well logging and the true character of the rocks penetrated. Furthermore, this project has shown that substantial scientific benefit can be obtained from continental drilling “holes of opportunity” if members of the scientific community are able to respond in a timely and coordinate

ISSN:0148-0227    

DOI:10.1029/JB088iB09p07267

年代:1983

数据来源: WILEY

摘要:

Geochronologic results are reported for 12 localities where deep drilling has penetrated Precambrian basement in Iowa, Illinois, Indiana, Michigan, and Kentucky. For the northern Illinois deep drill holes UPH 1 and UPH 3, a zircon U‐Pb age of 1463±10 m.y. was obtained. Rb‐Sr data for these samples indicate mild disturbance of this system but are not interpreted in terms of any specific younger event. U‐Pb ages for other localities fall in the interval 1450–1510 m.y. and indicate that this was a major period of igneous activity throughout the southern Great Lakes region. Rb‐Sr results indicate that alteration was ubiquitous and that most ages in the range 1100–1400 m.y. probably represent disturbed systems. The results obtained so far tend to support interpretations in which Kentucky, Indiana, and Illinois are primarily underlain by a rhyolite and epizonal granite terrane that is 1450–1500 m.y. old; older Proterozoic rocks may be absent or more

ISSN:0148-0227    

DOI:10.1029/JB088iB09p07276

年代:1983

数据来源: WILEY

摘要:

Two main basement granitoid types have been identified in core samples from the Illinois deep hole project. The main variety is a medium‐ to coarse‐grained porphyritic biotite granite with phenocrysts of microcline perthite and less commonly quartz and sodic plagioclase in a matrix of these minerals and biotite, muscovite, fluorite, magnetite, ilmenite, zircon, hornblende, apatite, sphene, monazite, rutile, and clinopyroxene (relict). The texture is typically hypidiomorphic gradational to recrystallized xenomorphic. The second main granitoid, which occurs in the upper part of hole UPH 3, is a fine‐grained granoblastic to lepidoblastic gneissic granite that is distinct from and possibly older than the nonfoliated granite. The textures of both rocks have been modified by a mild cataclastic shearing that has partially recrystallized the more susceptible mineral phases. Thin fracture planes that crosscut the earlier foliations are common. Microprobe analyses indicate that biotites in the gneissic granite are chemically distinct from those in the granite. Biotites in the gneissic granite have higher Fe/Fe + Mg ratio, FeO, and Al2O3and lower MgO and SiO2. Reflection microscopy and microprobe analyses indicate that the oxide phases in the two rocks are also different. The oxides in the granite are magnetite and ilmenite, whereas hematite and pseudobrookite occur in the gneissic granite. The biotite and Fe‐Ti oxide data represent additional evidence in support of the fact that the granite and gneissic granite are distinct rocks and probably not part of a continuous comagmatic sequence. Major element chemical analyses indicate that the granites have affinities to anorogenic rapakivi granites. The granites in the deep holes are high in SiO2, alkalis (Na2O+K2O), F, FeO/MgO; low in Al2O3, FeO, Fe2O3, TiO2, MnO, and P2O5; and slightly low in MgO and CaO. Magnetic susceptibility and density measurements correlate generally well with magnetic susceptibility and density logs. Magnetic susceptibilities range from 20 to 17,000 micro SI units; higher values correlate mainly with greater amounts of Fe‐Ti oxides and biotite; lower values with gneissic rocks and muscovite content. Density measurements range from 2.595 to 2.685 Mg/m3; denser rocks correlate positively with higher amounts of biotite and Fe‐Ti oxides and inversely with microcline perth

ISSN:0148-0227    

DOI:10.1029/JB088iB09p07287

年代:1983

数据来源: WILEY

摘要:

Seventeen whole rock samples and 27 mineral separates from the Precambrian granites in drill cores UPH‐1, ‐2, and ‐3 have been analyzed. The δ180 values for quartz remain exceedingly uniform (δ =7.6–8.6‰) throughout the entire length of the three cores and are indicative of their primary igneous values. The δ180 for the feldspars are also uniform for samples located 70 m or deeper below the unconformity (δ =6.0–6.9‰) However, in the upper 70 m of the granite body, a gradual upward increase of δ180 in the feldspars is observed (to 8.8‰). Similarly Δ18quartz‐feldspar show typical plutonic values (1.4–2.1) in the main part of the Precambrian sections except in the upper 70 m, where ΔQ‐Fbecome progressively smaller upward until a reversed fractionation (negative ΔQ‐F) is observed in samples several meters below the unconformity. The increase of δ180 in the feldspars is interpreted as a result of hydrothermal alteration of the granites at low temperatures (probably between 110°–260°C). The source of the hydrothermal fluid responsible for the alteration most likely derived from the formation waters in the upper Cambrian‐Ordovician sedimentary rocks overlying the unconformity. The alteration occurred at least 135–200 m.y, ago when there was also a lead mobilization event. The relative low primary18O/16O ratios in the granites suggest that the granites may have derived from partial melting of low

ISSN:0148-0227    

DOI:10.1029/JB088iB09p07300

年代:1983

数据来源: WILEY

摘要:

The granites sampled by the IDHP have whole‐rock δ18O values of about 7‰, which places them at the low end of the range previously observed for granitic rocks that not undergone extensive modification by hot meteoric waters. Over a depth range of 840 m, no significant variation in bulk initial isotopic composition is observed. All samples have experienced post‐crystallization retrograde isotopic exchange among the major minerals (quartz, feldspars, biotite), as is common in plutonic igneous rocks. In the uppermost sample analyzed (only 3 m below the Paleozoic sediments), feldspar has undergone exchange with an external reservoir, probably water of meteoric origin, and probably at a temperature near

ISSN:0148-0227    

DOI:10.1029/JB088iB09p07305

年代:1983

数据来源: WILEY

摘要:

A gravity study of northeastern Illinois and southwestern Wisconsin was undertaken to determine the three‐dimensional geometry of the Precambrian granitic body encountered in the Illinois deep drill holes. Gravity data acquired during this study and existing observations show a marked negative gravity anomaly in the vicinity of these drill holes which is interpreted as originating from a negative density contrast between the granitic body and the intruded crystalline rocks. A fifteenth‐order polynomial surface fitted to the Bouguer anomaly data (which includes only wave‐lengths greater than ∼35 km) was chosen as best displaying the effects of the granitic body, and profiles of this surface were modeled employing a variety of techniques and assumptions. The preferred model of the causitive body has a depth to the top of 2 km, a density contrast of −100 kg m−3, and a maximum vertical extent of 5 km. An isopach map of the body constructed from the modeling places the Illinois deep drill holes on the edge of an oval

ISSN:0148-0227    

DOI:10.1029/JB088iB09p07307

年代:1983

数据来源: WILEY

摘要:

The238U‐234U‐230Th‐232Th system has been investigated in 84 silicic crystalline rocks obtained from drill cores, surface, and near‐surface samples in California, Wyoming, Colorado, and Illinois. Results of these analyses displayed on ternary diagrams with apexes for238U,234U, and230Th indicate five predominant geochemical processes that affected uranium in the rock: (1) bulk uranium leaching where238U and234U were removed with little or no fractionation; (2) preferential234U leaching by alpha recoil displacement (234U recoil loss) with lesser238U loss; (3)234U recoil loss with little or no238U loss; (4) uranium assimilation where both238U and234U were added with present‐day234U/238U activity ratios varying from 0.8 to 1.2; and (5) addition of234U and230Th by daughter emplacement processes (234U +230Th recoil gain). Evidence for the existence of234U and230Th recoil gain in rocks is the most important finding of this investigation. Radioactive disequilibrium occurs in the majority of rocks analyzed where234U recoil loss is the predominant process associated with incipient weathering; U assimilation and234U+230Th recoil gain occur under conditions of substantial water penetration along fractures and into weathered zones in the rocks. Relatively unfractured and petrographically fresh rocks from the UPH‐3 drill hole in northern Illinois are closest to being in radioactive equilibrium for any suite of rocks included in this study, and they demonstrate that equilibrium during the last 0.5 m.y. can be maintained over a substantial vertical distance where there has been little or no movement of water in the basement rock. The238U‐234U‐230Th system can be a sensitive indicator of geologically recent U mobility and rock/water interaction both in petrographically fresh core samples and in ‘seale

ISSN:0148-0227    

DOI:10.1029/JB088iB09p07315

年代:1983

数据来源: WILEY

摘要:

The composition of groundwater in deep‐seated rocks is of great interest, both geochemically and in connection with the disposal of radioactive waste. However, sampling of deep‐seated groundwater is expensive and often difficult. We describe here a simple technique for the elution of pore fluid (groundwater) from cores of igneous rocks, and we describe brine eluted from a core of Precambrian granite from Northern Illinois drill hole UPH‐3. The brine is predominantly NaCl and CaCl2, with a Ca/Na mole ratio of about 0.05 to 0.1. From analysis of soluble chloride in a crushed sample the chloride concentration in the pore water is estimated as 2.3M. The brine is similar to brine in the Illinois Basin, and it was probably derived from the overlying sedi

ISSN:0148-0227    

DOI:10.1029/JB088iB09p07331

年代:1983

数据来源: WILEY

摘要:

The Pb‐Pb, Th‐Pb, and U‐Pb systems of whole‐rock drill‐core samples of the granite of UPH record at least one postcrystallization event. Treatment of the data on a concordia plot indicate the timing of this disturbance was probably 260 ±35 m.y. ago and probably no older than 400 m.y. ago. The upper intercept age on concordia is 1416±20 m.y and the Pb‐Pb isochron age for the four least disturbed rocks plus the four potassium feldspars is 1451±19 m.y. These ages are in close agreement with Rb‐Sr whole rock and U‐Pb zircon ages determined by others and should be the age of granite formation. Considerable Pb loss is indicated for many of the shallower samples that have a more red rather than pink color, probably to a factor of two to three or more as a result of the disturbance, although the granite remains Pb‐rich today (40–80 ppm). In some other studies of disturbed U‐Th‐Pb systems, the radiogenic lead has been taken up by potassium feldspars; however, the Pb for most samples seems to have been lost from the system for the granite of UPH. One sample with an unusually high Pb content of about 195 ppm has the most radiogenic Pb of any sample but ‘normal’ values of Th‐U. This sample has obviously gained radiogenic Pb (by more than a factor of four) and confirms that Pb was indeed mobilized in parts of the system. Within the limits of the uncertainties, equal amounts of Pb were lost or gained in both the U‐Pb and Th‐Pb systems, which is compatible with mobilization being restricted to lead. There is some tendency, however, for the more affected samples to have lower Th‐U values and higher U contents. A complex model in which U is gained, Pb lost, and208Pb lost preferentially to207Pb and206Pb cannot be ruled out. U and Th have somewhat different crystallographic locations and decay energies so that208Pb might be in more accessible sites than the other Pb isotopes. Fission track mapping of the samples indicates most of the U today is located in submicroscopic sites along edges and cleavages of biotite and iron oxides so U is readily available for remobilization. Lead extraction with only minor U or Th removal may have been accomplished by chloride‐rich brines. Lead isotope data on the granite of UPH indicates that some of the Pb in the ore in the nearby Upper Mississippi Valley lead‐zinc district may have been derived from these granites. An even better match is made with the ores of southeastern Missouri, although a granite like that of UPH would only be one end member (most likely the radiogenic end member) of a mixed source. Curiously, similar aged and heavy metal rich granites from southeastern Missouri are not a good match for theckn nearby ores. Basement rocks such as the granite of UPH must be reconsidered as an important contributor to the ores of the Mississippi Valley, but it is unl

ISSN:0148-0227    

DOI:10.1029/JB088iB09p07335

年代:1983

数据来源: WILEY

摘要:

Laboratory and well log physical property measurements show variations in the mineralogy with depth in UPH‐3. Gamma ray values generally decrease with depth in the drill hole, corresponding to a decrease in the felsic mineral components of the granite. Correspondingly, an increase with depth in mafic minerals in the granite is indicated by the magnetic susceptibility and gamma ray measurements. These mineralogic changes indicated by the geophysical well logs support the hypothesis of fractionation during continuous crystalization of the intrusive penetrated by UPH‐3. Two fracture zones and an altered zone within the granite penetrated by drill hole UPH‐3 are defined by the physical property measurements. An abnormally low magnetic susceptibility response in the upper portion of the drill hole can be attributed to alteration of the rock adjacent to the sediments overlying the granite. Fracture zones can be identified from the sonic velocity, neutron, and resistivity measurements. A fracture zone, characterized by low resistivity values and low neutron values, is present in the depth interval from 1150 to 1320 m. Low magnetic susceptibility and high gamma ray values indicate the presence of felsic‐micaceous pegmatites within this fracture zone. An unfractured region present from a depth of 1380 m to the bottom of the hole is characterized by an absence of physical property variations. The magnetic susceptibility and gamma ray measurements indicate a change in the amount of mafic minerals at the base of this otherwise homogenous region of the drilled interval. Abrupt changes and repeated patterns of physical properties within the drill hole may represent interruptions in the crystalization process of the melt or they may be indicative of critical temperatures for specific mineral assemblages within the in

ISSN:0148-0227    

DOI:10.1029/JB088iB09p07346

年代:1983

数据来源: WILEY