Limestone Xenoliths in Hills Pond Lamproite, Woodson County, Kansas

INTRODUCTION

Figure 1. Locality map for Silver City Dome and Rose Dome in Woodson County, Kansas. Detailed map of Silver City Dome adapted from Wojcik and Knapp (1990, fig. 1). X = site of xenolith discovery. Ps = upper Pennsylvanian Stranger Formation consisting of sandstone, siltstone, and shale. Click on the small image to see a full-sized version.

The intrusive rock of Silver City and Rose Dome was earlier referred to as mica peridotite or kimberlite. It is now recognized as lamproite, a potassium-rich, intermediate to ultramafic, alkalic rock with high K/Na and K/Al ratios. Intrusion of lamproite took place approximately 88-90 million years ago during the mid-Cretaceous, when the present surface was covered by about 1 km of overburden strata. The intrusions are hosted at the surface in upper Pennsylvanian strata of the Douglas Group consisting of clastic rocks--sandstone, siltstone, and shale.

The Hills Pond lamproite is located along a steeply dipping fault in the northern portion of a ring-graben structure that surrounds the central uplift of Silver City (fig. 1). The sill is up to 20 m thick and intrudes the Stranger Formation (Douglas Group), a clastic unit consisting of sandstone, siltstone and shale. Fossiliferous limestone of the underlying Stanton Limestone (Lansing Group) is exposed at the center of the dome structure, 1-2 km south of the sill. The lamproite includes major constituents of phlogopite (mica), olivine, diopsidic augite, and amphibole. These are found as phenocrysts in serpentinized groundmass with quartz and calcite veins. Surficial portions of the sill are weathered largely to clay minerals. Contact metamorphism of surrounding strata produced a narrow zone of hornfels-grade quartzite, slate, and skarn.

The distinctive mica-bearing rock at Silver City has been known since the late 1800s when it was the subject of a mining swindle from whence the site's name is derived. In recent decades, the Hills Pond lamproite has been mined for a variety of products used in agriculture and industry. The active surface mine has afforded the opportunity for field observations and sample collection by the authors during the past 20 years. We have visited the site yearly in connection with student field trips. Our efforts have focused on the sill margin and adjacent contact metamorphic rocks.

	Figure 2. Highly altered rock fragment from the Hills Pond lamproite. Small remnants of limestone (L) are visible surrounded by quartz-rich veins. Voids are "empty pockets" from which limestone remnants presumably were removed by near-surface weathering. Coin is 1 inch (2½ cm) in diameter. Click on the small image to see a full-sized version.
	Figure 3. Closeup view of limestone xenolith in which a fossil crinoid (C) stem is clearly visible. Coin is 1 inch (2½ cm) in diameter. Click on the small image to see a full-sized version.

The veins within the xenoliths are similar to those found in contact metamorphic rocks along the southern margin of the sill. The contact metamorphic rocks are marked by brecciation, deformation, and strong alteration of the original clastic strata, which consist of fine sandstone, siltstone and shale in the immediate vicinity (fig. 4). Secondary quartz mineralization is quite common in the contact metamorphic zone within 10 meters of the sill. However, the appearance of contact metamorphism disappears more than 30 m away from the sill.

Figure 4. Contact metamorphic rock from south edge of mine in Hills Pond lamproite. Note kink folds, brecciation, and strong alteration of the parent clastic strata. Coin is 1 inch (2½ cm) in diameter. Click on the small image to see a full-sized version.

Other, slightly deeper Pennsylvanian limestones of the Lansing and Kansas City Groups are considered possible sources as well. Another, still deeper source for limestone xenoliths could be Mississippian or Cambro-Ordovician strata. These rocks consist of thick dolostone and cherty limestone beds, and chert of presumed Mississippian age is common among xenoliths at Rose Dome. The top of the Mississippian lies about 500 m deep at Silver City (Wojcik and Knapp 1990).

We consider a Mississippian or deeper source for xenoliths at Silver City unlikely for two reasons. First, no chert or dolostone was discovered along with the limestone xenoliths at Silver City. The lack of chert or dolostone argues against a Mississippian or Cambro-Ordovician source for the xenoliths. Our second argument concerns temperature of the intrusive magma. Intrusive temperature at Rose Dome is considered to be >800°C, which was hot enough to cause metamorphism and partial melting of the granite xenoliths (Franks et al. 1971). Assuming a comparable temperature at Silver City means that limestone would be melted readily during transport in the magma. However, the xenoliths show no evidence of metamorphism, recrystallization, or partial melting. Limestone survival from a depth of half a kilometer to the xenoliths' present position seems improbable.

Our prefered interpretation is that limestone xenoliths were derived from shallow Pennsylvanian sources within the sill complex. Genesis of the xenoliths began as quartz veins were precipitated in highly fractured wall rocks. Some fragments of the wall rocks were detached, transported a short distance (10s of meters), and emplaced at the rapidly cooling margin of the sill. Those limestone fragments that were transported for longer distances or toward the center of the sill were totally melted.

The Hills Pond lamproite contrasts with the lamproite at Rose Dome where many xenoliths are present. Luczaj (1998) interpreted the latter as an explosive igneous structure. The general lack of deeply derived xenoliths in the Hills Pond lamproite suggests that intrusive movement was insufficient to carry rock fragments into the sill complex. This interpretation supports the notion of Wojcik and Knapp (1990) that magma intrusion at Silver City was a quiescent phenomenon.

ACKNOWLEDGEMENTS

Related sites